I think that Homer drives a Volkswagen Fox.
Think about it, it's boxy like a Fox, and in the episode where Homer invents the tobacco-tomatoe plant - when he's at the store buying seeds a couple of hill billies are making fun of him for being a city slicker, one of them comments on Homer's fancy "european" automobile, and Homer replies that it was built in Guatemala.

So apparently, he drives a car with a European brand, that was manufactured in South America - and it's boxy. What else would it be other than a Fox?
O.K. so he says Guatemala rather than Brazil - but on the Simpsons they wrongly identified a Commodore computer as a Coleco computer, so it's close enough for me.