Since instant tank controls (sorry I don't remember how this feature was named) were added it is almost impossible to control the tank when you're the gunner with AI driver and have mouse steering enabled.
If "Car left" and "Car right" are set to "mouse left" and "mouse right" when player is the gunner, turret rotation causes tank turning as AI receives mouse movements as turn command. Also AI will turn the tank even if "alt" is held while player rotates the camera.
Basically since tank controls update, mouse steering is almost unusable with tanks. I think it would be reasonable to disable the mouse steering when player sits in turret. (This style of controls was working fine since OFP)