Using the foreach command in your trigger would probably be easier to use instead of a script.
"_x moveincargo car1" foreach units group c1
But if you insist on using a script, I think your order of the arguments is incorrect.
[[unitname, unitname], car] exec "cargoload.sqs"
In your sample, you have the units to be put in cargo as the first argument and the car as the second argument. But in the script, you have that the car is the first argument and the units to be put in cargo are the second argument. I hope this made sense.
UberKreb