Spare parts sales.

[Sarchez]

This could create a secondary income stream, as you service/sell replacement parts - for the vehicles you've sold years ago. (Current rating of an old car, could come into play with frequency you are selling parts.).

I know this opens up for setting prices for spare parts (or atleast an index figure), that should have an effect on sales of new cars. (High sparepart prices, might scare some people off and vice versa.)

[Eric.B]

I am planning on adding parts/brand licensing income eventually. It's just not in the works yet because I need to figure out how to charge more expenses!

Game is too easy as it is...

[Frankschtaldt]

This would be an awesome inclusion...
... and perhaps should be geared as a way to offset the expenses of a guarantee rather than a new way to make more money. Cuz, as you say, we don't really need that.