Thanks for the reply Snefens.
Just so all the new folks (like me) know, this was basically a key-mapping issue for me. The trick is that the turret in the panzer appears to be a "gunner" position, just like in a bomber. The co-ax is the secondary weapon in the turret...So if you set your "gunner" controls to "fire secondary" you'll get it. Note that I said "gunner" not "vehicle." I think this is what threw me off.
CU up,
Catfish