I posted this in the electrical forum but there are a whole lot more folks in here!
When I connect the flat four connector from my boat trailer to the truck, everything works except for the running lights (turn signals/brake lights work fine). It is not the trailer because I hooked up to another vehicle and everything works fine. Anyone have an idea? I thought fuse? I checked the fuse in position 10 under the dash and it is fine. I looked at the relays under the hood, there is no relay in R201? Could that be it? Please chime in with any ideas. Thanks in advance!!!

 

Sounds like it's the connector on your truck. Try testing the voltage from that pin with a meter or test light. Might be just a wire came out of the socket or loose. You have a good ground, otherwise nothing would work. Really is simple to check with a test light. You should have power to the top left pin for running lights if it's a six pin connector with a connector in the middle. See if you got voltage there, if not, start looking upstream to find the problem. I just checked mine for the heck of it and had power at that pin when I turned on the parking lights. Yours may be different. fyi

Trailer Tow
The trailer tow stop/turn lamps receive power from the vehicle's rear stop/turn lamp circuits. When a rear stop/turn lamp is illuminated, voltage is also sent to the trailer tow connector. The trailer tow parking lamps are supplied voltage by the trailer tow parking lamp relay. When the parking lamps are illuminated, voltage is also sent to the trailer tow parking lamp relay coil. When the trailer tow parking lamp relay is energized, power is routed to the trailer tow connector. The trailer tow reversing lamps are supplied power from the vehicle's reversing lamp circuit. When the reversing lamps are illuminated, voltage is supplied to the trailer tow connector.