another thing you can do is (this was taught by macguba to me):
select activated by anybody present
in the condition field of the trigger put:
this && "land" counttype thislist > 0
and in the on activation field put
explo1 = "Heat125" camcreate [getpos (thislist select 0) select 0, getpos (thislist select 0) select 1, 0.1]
this will blow up any land units (men, armour, APCs, cars, Trucks) when they get in to the trigger.
or if you just want cars to blow up change the "land" to "car" which will blow up all jeeps, cars and trucks and the tractor but won't touch anything else.
Oh and don't forget to set it to repeatedly, otherwise it will only happen once and won't happen again. unless that's hat you want. hope this helps.
ryan out