As long as the truck has a name, this is very simple:
In the trigger's Condition field, simply write:
playerName in truckName
Simple as that! The trigger activates when playerName is in truckName. Alternatively you can use just "player", but you should avoid that unless absolutely necessary, because you might for instance want to test the mission from another perspective and let the AI handle the player's tasks - in which case the trigger would never trigger and such.
Here are some alternative conditions:
vehicle playerName == truckName
or, if you want to wait until the player starts the truck:
isEngineOn truckname && vehicle playerName == truckName
The placement radius doesn't make any difference
Wolfrug out.