Enter the following codes for the corresponding cheat. 
 

Code:                         Result:
MUS59279                 All Muscle Cars
TUN58396                  All Drift Cars
M38572343                Micromania Pagani Zonda R
P47203845                 Play.com Astom Martin DBR9
G29782655                Gamestation BMW 320si
F93857372                 Buchbinder Emotional Engineering BMW 320si

Disable Opponents
Go to your Grid Folder and then the AI folder. Remove the file \\\ "ai_vehicle_config \\\". If you start a race, the opponents stay and you can enjoy the race. Make a backup of this file before removing so you can restore the game's original functionality.