Have an '04 chevy suburban. Problem: Dual climate control, front/rear hvac units. Rear unit will not switch from hot to cold and vice versa. Actuators (2) both work, door flap moves freely, no binding. Tried the "Remove the fuse and reset the system" No change. Appears Aux. heat control module needs replacing. On the web, this module is mounted between the two actuators. Not on mine- no sign of it. So....Where the heck is it??