Depending on when you want the bomb to explode, there are a few ways to do this. If you want the bomb to explode the instant the civilian gets into the car, then you can use SeAnVeR's code and put that in the civilian's Get In waypoint's activation field.
If you'd rather have a slight delay for some added realism (since most car bombs don't go off just because you got in the car, but rather because you started the car), then you can apply a slight delay. In the waypoint's activation field, have it execute a short script that has the following code:
; Delay for 1 second.
~1
; Create an explosion.
_tempObj = "SHELL120" camcreate car
Where car is the name of your car. Replace SHELL120 with whatever explosion type you really want to use.
By changing the delay, or by having a different waypoint create the explosion, you can alter when the car bomb will go off, if you so desire.