Enter the following cheats at the 'Options' menu:

All Cheats Activated:
CHEATINGMUTHATRUCKER or ATJCEHJMJ

All Missions:
LAZYPLAYER

Automatic Sat nav:
USETHEFORCE

Big Money:
LOTSAMONEY

Diplomatic Immunity:
VICTORS

Disable Damage:
6WL

Evil Truck:
VARLEY

Hyperfast Trucks:
GINGERBEER

Small People:
DAISHI

Extra Levels:
JINGLEBELLS

Unlimited Time:
PUBLICTRANSPORT

Thanks to Revolution readers TroJax, sam and Barton!