Location of ground wire for 2008 X5 trailer package
 

Having issues with my trailer package. Started about six months ago when the truck was telling me one of the lights on the trailer was out...it was not. Then when in reverse it would not recognize that the trailer was hooked up thus the backup sensors went off. My first move was to completely check the trailer...everything checked out on the trailer side. I am thinking it may be the ground on the truck side...don't know where to look for it. Any suggestions? Thx