Copyright illustration of car in logo

This is indeed a legal question, but here's my take as an artist/designer:

If it's an illustration that you, yourself created from scratch, and you are not in any way using the trademarks of that brand, you are in the best situation, as that's an original artwork. For instance, I don't need permission to paint a painting of my own car. But that's not the same as using the image for commercial purposes so this is not in any way saying it's legally safe. e.g. one could get in trouble is if your client is misrepresenting the brand of car in any way. If customers think Porsche is endorsing your client because they have a representation of a Porsche in their logo, Porsche will definitely have something to say about that. Regardless, your client would never ever be able to trademark this logo, so why even risk going there.

If your client insists, you need to consider legally protecting yourself. I doubt a waiver ("the client made me do it") would be of any protection from Porsche 😉

Without even knowing where you are from or what car this is, this is impossible to answer even in the most superficial way. Trademark and copyright issues are complicated. The only clearcut case I can imagine for using a recognizable car illustration is pretty much a car dealership where they sell cars from this manufacturer and would tolerate or even endorse the logo use, but everything outside that is a legal mine field.

Mylenium

Truthfully, the best answer is to always play it safe. Either get permission or use something generic. Even with a contract that seems to protect the designer, I have seen companies go after designers because they should know better. So even though any monetary rewards were paid by the client ultimately, the designer's name was sort of dragged through the mud so to speak. Perhaps you can use the car but change it enough to satisfy the client but not be identical so you don't get in trouble.

https://www.nolo.com/legal-encyclopedia/when-you-need-permission-use-trademarks.html

Like a lot of legal questions, I believe the answer is "It depends."

It depends on the regulations in the territory you live in, the recognisability of the vehicle, and possibly other factors like how it will be used.