so, as many before me, my tailgate will not lock or unlock. I took apart the rear hatch and started to inspect the actuator for anything broken. Everything looked good, however, i seem to be missing the 3rd plug in. I looked at ebay at used ones and it seems that a red wire and a brown wire should be going into this plug. Is that the case? i search high and low for a missing plug. I bought the car used and can tell somebody has been behind the panel before. I maybe just chasing my tail so i am hoping somebody can help me. 2003 ML350 w163

 

The previous owner probably disconnected the plug because the lock was machine-gunning. Plug in the Pink & Brown connector and see what happens.

If it machine guns then in all probability, the button on the lock actuator has broken off.