I have completed the navigation install and everything works great...but I just never got around to connect C5 from the nav unit which connects to the tail light harness (Back Lt). Does anybody know what this actually does?

 

Probably tells the navi brain that the car is either stopping or slowing down. im sure its so the navi can calculate a more precise position to show on the screen.

orrrrr maybe its for the reverse light. to tell the navi that the car is backing up. again, to get a more precise reading.

 

have u tested the unit without it plugged in??? maybe that will give you a clearer indication of what it is used for... (i.e. if you are in reverse and it is not moving on the screen)... im pretty sure i had to hook up a reverse wire or was it a brake wire, on a aftermarket Pioneer Navi but i don't remember what it was for... good luck