crazy as a pet coon under a red wagon
It seems your brother-in-law is mixing up two more common sayings -
Crazy as / crazier than a pet coon: a much more common saying meaning a crazy/demented person. (Also related: Lazier than a pet coon) References can be found on on urban dictionary, wiktionary, and quite a few other sites. It's agreed to be a Southern/hillbilly colloquism.
and
Cute as a speckled pup under a red wagon: another much more common saying meaning pretty cute. References can be found on various sites again. Seems to be from the same region as the above coon saying.
For theories of etymology, as Sven points out, this half of the phrase could itself be a mixing up of two different crazy metaphors -
- crazy old coot (a water-fowl whose name was used for silly people/fools since the late 1700s at least)
+- crazy loon (short for lunatic; or moon-touched, which was believed to cause madness; and also yet another water-fowl who apparently had a crazy cry.
But the word coon did refer to one thing other than a raccoon during this time period as well. So the phrase could actually be referring to a:
Coon: insulting U.S. term meaning "black person" was in use by 1837, said to be ultimately from Portuguese barracoos "building constructed to hold slaves for sale."
No doubt boosted by the enormously popular blackface minstrel act "Zip Coon" (George Washington Dixon) which debuted in New York City in 1834. But it is perhaps older (one of the lead characters in the 1767 colonial comic opera "The Disappointment" is a black man named Raccoon). [Source]
Strengthening the assumption of a Southern etymology, an early example of crazy coons is from the reported speech of a Kentuckian, as written by a temperance advocate from Boston in 1873. Another is in a Methodist Magazine, though one published in Canada!
Unlike the old crazy coons, however, the cute pups only seem to have arrived on the scene in the 1900s and gained popularity mid-century. It appeared in a list of similes printed in the Tennesee Folklore Society Bulletin in 1950 and examples start cropping up more frequently in the 60s and some in the 70s, but only a few odd mentions before the 50s.
So it seems likely that your bro-in-law is just mixing up these two more common sayings for his own personal "saying".
Maybe it came straight out of your brother-in-law's imagination; or maybe he just gave a twist to "cute/purty as a speckled pup under a red wagon".
Whatever its origin, it doesn't appear to be recorded in any online source other than the various places you have asked the same question. This strongly suggests that it is unique to your family or the local community in which you were raised.
My answer merely supplements the useful and succinct answers provided by Erik Kowal and Shisa with a bit more historical and contextual detail.
The Phrases “Crazy as a Raccoon” and “Under a Red Wagon”
The “under a red wagon” part of the OP’s phrase comes from various similes dating to the early twentieth century that consistently involve dogs.
A Google Books search turns up several variants. From C. Z. Hartman, “A Chef From New England” in Table Talk (October 1904):
He was the only one that tuk any notice o’ me, an’ I must say he treated me han’some, considerin’ how he must a’ ben yearnin’ to jine th’ crowd around Camelyer. Not that I blamed him much, fer she's ez purty ez a speckled pup under a red wagon ; an' pop'lar — w'y sassiety jes' couldn't git enough of her!
From Harr Wagner, “Combining Business and Pleasure” in Western Journal of Education (1918) [snippet]:
Were you ever tired of your desk? Tired of your routine? Tired of your best friends? Tired of your environment? Well, if you ever were as tired as six yellow dogs under a red wagon, then you will appreciate my condition when I turned my face away from the idlers in Union Square, from the sight of formal luxury of the St. Francis and my back on the office, and took the train for somewhere.
From Theron Sedgwick, York County, Nebraska, and Its People (1921):
York was all “dolled up” to receive her visitors, too. The courthouse was decorated with strings of flags and looked as pretty as a spotted dog under a red wagon.
The only instance of “crazy as a raccoon” that a Google Books search turns up is this one, from Lucius Sargent, “Margaret’s Bridal” in The Temperance Tales (1853):
“Stranger,” said a raw-boned Kentuckian, who had listened in silence for some time, “both on ‘em’s the devil’s work, I tell ye. I 've tried 'em all, and been jest as crazy as a 'coon with a slug in his ear, 'pon every one on 'em, from streaked ale up e'enamost t' akyfortus."
However, a Google Books search for “crazier than a” turns up three fairly recent raccoon-related items. From Peter Davies, Storm Country (1992) [snippet]:
Wayne said Sharon's parents were "crazier than a pet coon. If they turn up, it's gonna be fist city round here.” He had, he said, thought seriously about packing his Colt.
From Ann Rinaldi, A Ride Into Morning: The Story of Tempe Wick (1995) [snippet; text not shown in linked pages]:
I am most saddened to hear of the death of Uncle Henry, for I was always fond of the old gent, although sometimes I thought him crazier than a coon. You know all the Wicks are.
And from Don Winslow, Way Down on the High Lonely (1998) [snippet; text not shown in link]:
"You don't look crazier than a pet coon!" Steve shouted. "What?" Neal Carey shouted over the noise of the old pickup truck as it rattled over Highway 50. "1 said you don't look crazier than a pet coon." answered Steve Mills.
As I detail in the next section of this answer, two of the most common “as crazy as” similes in U.S. English are “as crazy as a loon” and "as crazy as a coot.” It seems at least possible that, sometime in the past few decades, speakers may have mashed the two birds together to produce “as crazy as a coon,” and then subsequently made the coon a pet.
In any event, as both Erik Kowal and Shisa note in their answers, the OP’s brother-in-law seems to have combined the dog-centric “under a red wagon” with a raccoon-centric version of the unrelated “as crazy as a __” simile.
Other “Crazy as” Similes
Frank J. Wilstach, Dictionary of Similes (1916) lists several then-standard “as crazy as” similes:
Crazy as a loon. — Anon.
Crazy as a woman’s watch. — Ibid.
Crazy as a bedbug. — J.R. Bartlett’s “Dictionary of Americanisms”
Crazy as a June bug. — William Allen Butler
J. D. Clark, “Similes from the Folk Speech of the South: A Supplement to Wilstach’s Compilation,” in Southern Folklore Quarterly (1940) adds these similes [snippet]:
crazy as a bat
crazy as a coot
crazy as a jackass
crazy as a lunatic
crazy as a madman
crazy as a nut
This source also has “cute as a speckled pup under a red wagon,” in a separate entry.
And New York Folklore Society (1973) weighs in with these [snippet]:
as crazy as a loon, a betsy bug, a bat
This source also has “as pretty as a picture, a peach, a pup under a red wagon, a speckled pup,” in a separate entry.
R. H. Thornton, An American Glossary, volume 1 (1912) dates “crazy as a bedbug” to 1861 and “crazy as loons” to 1848. Thornton also notes a further variant:
Crazy as a bedbug. This ungenteel simile is occasionally varied by calling the insect a “Kalamazoo bedbug.”
Among the more colorful “crazy as a” similes in a Google Books search are these:
crazy as a bachelor tending a crying baby (1854)
crazy as a Bedlamite (1834)
crazy as a box of birds (1997)
crazy as a Cape Cod clipper (1840)
crazy as a creek crane (1984)
crazy as a fly in a drum (1830)
crazy as a Gadarene swine (1989)
crazy as a goat in the midsummer sun (1955)
crazy as a goose [or a sheldrake] shot through the head (1830)
crazy as a herring buss in a storm (1841)
crazy as a horse in a windstorm (1940)
crazy as a hoot owl (1947)
crazy as a jay bird (1948)
crazy as a locomotive (1875)
crazy as a mad rooster (1834)
crazy as a mangy dog (1700)
crazy as a man with monkeys after him (1856)
crazy as a nut in a peach orchard (1947)
crazy as a painter (1907)
crazy as a parrot eatin’ stick candy (1971)
crazy as a peach orchard pig (1947)
crazy as popcorn on a hot stove (1971)
crazy as a rat in a drainpipe (1980)
crazy as a rotten post (1687)
crazy as a sheepherder — (Cattlemen's expression.) (1939)
crazy as a shitepoke (green heron) (1939)
crazy as a wounded bear (1839)
The “crazy as [or crazier than] a pet coon” simile is evidently much more recent than most of these alternatives.