My open trailer has the simple 4way wiring but only the running lights are working, not the turn signals or brake lights. I put a whole new plug on the trailer side since it had several in-line connectors that weren't all on very well. I also put and brand new plug on the truck side. Still doesn't work. All trailer light bulbs work except for one of the 3 middle lights thats bellow the tailgate. I looked at all the wiring underneath trailer and nothing appears to be damaged. Does the truck have a problem or is in the trailer? As far as i know the truck is fine because it works with my father's encl. trailer that has the 7way wiring. Anybody got knowledge on this? Thanks.