Pause the game and select the "settings" option. Then, select the "cheats" option and enter one of the following codes to unlock the corresponding cheat option, which can then be toggled on or off:

Invincibillity: IRONMAN
Indestructable cars: ROLLBAR
All vehicles: CARSHOW
Free garage: TOOLEDUP
Infinite nitro: ZOOMZOOM
All weapons in era: GUNRANGE
Infinite ammo: GUNBELT
Weaken the cops: KEYSTONE

You will notice there are more gaps let me know when you find out what they are Thanks.