when i use the tank in commandos behind enemy lines, the tank will drive but wont fire, ive looked online for alternate controlls but i couldnt find anything, when i press ctl + click it does the fire sign but it doesnt fire

The fact that the cursor turns into a cross hair confirms that you're doing it right - left clicking when this is the case should do the trick.

My question is though, what kind of tank are you talking about? Are you completely sure that the vehicle you're talking about is able to shoot? What mission was this on? (although I guess that by now you've figured it out anyway)