Lesman reminds me...
I have had bad problems with corrosion on the trailer connector each year on my tow vehicle. (They use too much salt on the highways in Wisconsin.) A good blast of WD40 seems to flush out that blue/green corrosion. This year I used WD40, followed by dielectric grease and my connections have been at their best. Hopefully the dielectric grease will prevent more corrosion this winter on my truck.
My solenoid is currently shot, so the brakes engage in reverse. I use electrical tape to hold a bolt in that slot on the trailer tongue where the surge brakes engage. That lets me back up, but I've got to remember to remove the bolt, otherwise I've got no brakes on the trailer. I've checked the electricity and power is getting to the solenoid, so I've got to take it over to Tom's for a repair sometime.