The car is pretty slick/sleek

Solution 1:

Slick, when it is used metaphorically instead of literally, simply means "cool" or "neat" or "clever", as shown by these definitions:

3 a : characterized by subtlety or nimble wit : clever; especially : wily • a slick swindler
b : deft, skillful • a slick ballplayer
4 : extremely good : first-rate
_{from m-w.com}

As you can see, there's nothing pejorative about it when used in this sense.

"Slick" can refer to the car's coolness factor; it covers how good it looks, how well it works, and how much nifty stuff it can do.

"Sleek" can only refer to the car's appearance.

Solution 2:

Per the OED, slick and sleek as adjectives both share a quite similar meaning, having both derived from Middle English slike, meaning essentially "smooth."

The pejorative nature of "slick" is much more recent and manifests in a slang noun form first attested as recently as 1959.

U.S. slang. A clever or smart person; a cheat or swindler. [emphasis mine]

This sense is cross-referenced to slicker and slickster, which means essentially "swindler." It is likely a figurative outgrowth of the earlier U.S. sense meaning "slippery."

The sense you refer to in the movie is more likely OED adjective definition 5.

First-class, excellent; neat, in good order; smart, efficient, that operates smoothly; superficially attractive, glibly clever. (Of things, actions, etc.)

This sense is first attested in 1833 making it also quite recent relative to more literal sense of the word slick, as in "smooth." Unlike the slang sense referenced first, there is nothing necessarily pejorative about this sense, though it is worth noting the second portion of the definition, "superficially attractive." This is where context comes in. In the context of a person referring to their own car as slick, it is unlikely that they meant "superficially attractive;" rather they likely meant "first-class, excellent."