When you dump a car at the car-dump, you'll get a power-up. The following is a list of cars with their corresponding power-up: 
Aniston BD4-Jail Free Card 
Beamer-Flame Thrower 
B-Type-Invulnerability 
Big Bug-Machine Gun 
Bug-Machine Gun 
Car-Power-Up 
Dementia-Invisibility 
Miara-Rocket Launcher 
Minx-ElectroFingers 
Michelli Roadster-Machine Gun 
Police Car-Cop Bribe 
Romero-Armor 
Schmidt-Health 
Shark-Molotov Cocktails 
Taxi-Double Damage 
U-Jerk Truck-Machine Gun 
Z-Type-Silenced Machine Gun 

Sent in by Kjetil "ABF" Engen

To activate cheat mode you need to first call yourself... gouranga If you've typed it right you'll get a low confirmation sound.
Then just call yourself: 

GOREFEST - Enable Blood Splatters 
BEMEALL or UKGAMER-All 3 cities 
EATSOUP - Free Shopping (For Car Upgrades etc.) 
GODOFGTA - ALL Weapons (Max Ammo) 
MUCHCASH - Gives you $500,000 (don't need activation code, but you're name must be muchcash all the time) 
BUCKFAST - Starts a city wide riot 
RSJABBER - God Mode or all guns? 
MADEMAN - All Respects from three gangs 
MEATMAN - Hamster runs across screen 
LASVEGAS - Some of the people turned to Elvis clan 
NEKKID - All people naked 
CUTIE1 - 99 Lives 
BEEFCAKE-Increased brutality 
ARSESTAR - Keep weapons after death 
DANISGOD - Gives you $200,000 
FLAMEON - Gives you Flamethrower (Unlimited ammo) 
VOLTFEST - Gives you Electro gun (Unlimited ammo) 
IAMDAVEJ - Gives you $9,999,999 
SEGARULZ - Gives you 10 Bonus Multiplier 
UKGAMER - Gives you all three levels 
TUMYFROG - Gives you all of the Bonus Levels 
SCHURULZ - Double damage powerup 
HUNSRUS - Invisible 
FISHFLAP - All cars become real small, and the music is play quick