Basically, when making the turret, think of it as making a playertype - 'cause that's what you're doing. Create a mount point for gunner, muzzle point for gun, animate the turret, and set up the turret script. Test it first as just a playertype. Once it works as a playertype, edit the script of the vehicle to mount the playertype on creation to one of the mount points of the vehicle (look at the tank script, for example).