maybe:
make a script and call it MG.sqs. The write in it :
#mg
MG setpos getpos Car
MG setdammage 0
Car setdammage 0
goto "mg"
Now write in a units init field: [] exec "MG.sqs"
But if somebody let explode grenades on it, the car won`t get damage.
;D
hope, this will help you.
Cheers for now