Information was from GameShark Gabe, at http://www.railway.org/gt/gtcars.html, but extensively modified in format.

Manufacturers' Races
South City
ManufacturerRaceNUPDoneComments
Acura None  
Chevrolet Corvette Meeting - Normal N    
Corvette Meeting - Racing N    
Dodge Viper Festival of Speed - Normal N    
Viper Festival of Speed - Racing N    
Neon Trophy - Normal N    
Neon Trophy - Racing N    
Plymouth None  
Shelby None  
Ford Ka Challenge - Normal N    
Ka Challenge - Racing N    
Focus Challenge - Normal N    
Vector None  

"N", "U", and "P" stand for "New", "Used" and "Prize" respectively, and indicate where you can or must find the car. I might get it entirely filled in some time. The important ones are ones where "N" is not available, but you must either find the car used, or win it as a prize.

Note that an additional aggravation is that very often prizes which it seems should be eligible are not. For example, the Mine's R34 Skyline GT-R cannot enter the R34 Race (although it can enter the GT-R Race!?!?!). (And it really cleans up, I might add )