Cheat Codes:
This requires you to modify a game file. Proceed at your own risk, and remember to make back-ups.
Edit the CONFIG.CFG file in the game directory with Notepad or a similar text editing program. Find the line "uset g_console" and change the number 0 to a 1. This will switch on the debug/game console. Save the COINFIG.CFG file.

Start up the game, then press the Tilde (~) key to display the console when you're in the game. You can now type in the following cheats:

To change the truck, use one of these:

Cheat truck 1
Cheat truck 2
Cheat truck 3
Cheat truck 4
Cheat truck 5
Cheat truck 6

To change your rating, use:

Cheat skill 1
Cheat skill 2
Cheat skill 3
Cheat skill 4

For other effects, use:

Cheat money X (replace X with the amount of money you want)
"Cheat dealers -- Shows you all the truck dealers on the map.
Cheat drivers -- Gives you all drivers.
Cheat stars -- Gives you all stars.
To resume play and close the console, just hit ~ again.