If grounding the sled wire doesn't do the trick, then either that wire has pulled out of the sled wiring loom, or you aren't using the right wire on your car's docking connector.

Don't just poke around with a test-light or a multimeter and "assume" a given wire does a given thing. The car wiring looms are very deceptive in this way and you can make things go badly if you do not use the right connections. Do not guess as to the functions of the wires on your car wiring loom, get a proper harness adapter from a place like Crutchfield so that you are absolutely certain.