"Left" and "right" are to "side" as "front" and "rear" are to what?

Solution 1:

End

Like "two ends of a stick"?

Solution 2:

In terms of anatomical locations, front (anterior) and rear (posterior) are on the anteroposterior axis.

So you could say "Which end of the Anteroposterior axis?" This is obviously ludicrous, but might be OK if your customer was an MD or biologist.

Solution 3:

I don't think so. You'd probably get 'front or back' again from some people, or just 'one' as in context, you've only got two bumpers, front and back.

You damaged your bumper? Ok, which one?

Solution 4:

Technically, side can include front, back, top, bottom, and more depending on how many sides an object has.

side noun
1. one of the surfaces forming the outside of or bounding a thing, or one of the lines bounding a geometric figure.

side
3 b : a line or surface forming a border or face of an object

[EDIT]
The import of this answer is that side is not incorrect, and as such we need look no further in the first place. I do not necessarily disagree with other answers, other than that they are oblivious to this fact.

The question is based on the presumption that side excludes front and rear, which I contradict here.