Heres where I would start. 1. Make sure you have a good ground wire connection. The white wire on the harness needs to go to vehicle ground on the truck side, and to the fram on the trailer side. A dangling/loose ground can produce lots of odd behavior as you depend on a ground thru the hitch/ball which is not very solid. 2. For the turn/stop light that wont work, take an ohmmeter and make sure there is a zero ohm connection from the correct terminal on the connector at the hitch to the lamp socket that wont work. If this is bad, then you have to fix whatever is preventing continuity. 3. Once that is OK, check the ground. Use the ohmmeter to measure resistance from the lamp receptacle on the back, ground connection, to the ground wire at the hitch connector. If it isnt 0.0 ohms, fix it. Often each lamp has its own ground attached to the trailer frame and these can rust, get dirty, or come loose. No ground means no light. As a last resort, make sure you have 0.00 ohms from lamp ground to negative battery post on the truck. If not, there is an open somewhere that has to be fixed, most likely where the wiring connector is grounded to the frame which can also come loose or rust up under your truck frame where it is not so obvious. Thats all there is to it. Or you can just pull all new wire which you know will work when you finish, if you prefer to not do any significant diagnostic work.2008 Pantera Classic2014 Mercury Pro XS 200
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