Enter the following codes for the corresponding cheat.
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
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.