Hello, as ASTTrprDillon95 write you have, to have a savegame with no cars in the save parking slots. So if you mod a car and its parked a space i will crash.
I found a 100% savegame with no cars in the parking spaces, and it works like a charm.
Just put in Documents/Rockstar Games/ Gta IV / Save games.
Link:
http://download.gta-expert.it/gta-4/salvataggi-pc/11251-gta-4-savegame-no-smpa60.html