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

Manufacturers' Races
North City
ManufacturerRaceNUPDoneComments
Aston Martin DB7 Trophy - Normal N    
DB7 Trophy - Racing N    
Audi TT Challenge - Normal N    
TT Challenge - Racing N    
BMW 3 Series Cup - Normal N    
Jaguar None  
Lister None  
Lotus Elise Trophy - Normal N    
Elise Trophy - Racing N    
Elan Trophy - Normal N    
Elan Trophy - Racing N    
Mercedes-Benz SLK Trophy - Normal N    
SLK Trophy - Racing N    
Mini-MG Mini Challenge - Normal N    
Mini Challenge - Racing N    
MGF Challege - Normal N    
MGF CHallenge - Racing N    
Opel Tigra Cup - Normal N    
Tigra Cup - Racing N    
RUF None  
TVR Tuscan Speed - Normal   P  
Tuscan Speed - Racing   P  
Volkswagen Golf Cup - Normal N    
Golf Cup - Racing N    
Beetle - Normal N    
Beetle - Racing N    

"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 )