Loosing trailer brakes

I had this before on my truck, the message has flashed up on my screen before. There are a few things that can cause this. There has been a post a few months ago on the same subject and I hope that OP can respond to what he had found. Mine has come on after it rains so I think that water gets in the rear control box or just a connection. I have not replaced anything because it goes back to normal. Google up what others have found, and the box by the spare tire seems to be a possible problem. I ordered one of those off eBay for around 70.00 just to keep if that's the case, cause the dealer charges alooooootttttt more..... Also get under neith the truck and follow the wiring harness, u may find a spot of rubbing or where water can enter. Hope this helps, and others will chime in.