The trailer lights on my pickup appear to be factory and I was having issues today when I was testing my lighting on my trailer. The trailer lights are all wired correctly as my neighbors truck tested it and it is good. When I hook up mine the left turn signal is not working at all. I tracked the yellow wire down and from the plug towards the dash is a black plastic box with a yellow and also a green wire going into it and a green, yellow and red coming out going towards the dash. The yellow wire has weak voltage on the plug side and a strong voltage on the dash side. This box is causing the droppage. What is this box? It is black and about 4"x3"X.5". I don't want to take it out because it must be doing something? Please help because I am tempted to bypass this by taking it out but that extra red wire is going in it. Green is right turn signal and yellow is left turn signal. Thanks.