ammo = "rhs_reammobox" createvehicle (getmarkerpos "marker_1")
I made a quick test and this worked fine for me. Are you sure you have a marker of that name? Are you sure the trigger is being activated?
The problem I had was that it was necessary to wait 20 seconds after creating the crate to access its contents. I think Aldo is on the right lines: place the crate in the mission editor first, name it, and then use setPos in the trigger to move it where you need it.
GrpNull is something that the game understands on its own, so you don't have to define it. The same goes for
player and
nil.