Windshield wipers not working

Hello, i am having some trouble diagnosing the non functioning windshield wipers on my car listed above z51. it has 32,000 miles and i have had zero issues. Then a couple weeks back the windshield wipers started not to work i would go for a drive and they would work and 15 minutes later they would stop working.. i would reset the car by removing the negative battery cable and the wipers would come back on. a week goes by and now they do not work at all.
I took it to the local dealer and they told me the under hood fuse box needed to be replaced.. the tech could push down hard on the wiper motor relay in the fuse box and the wipers would work. so he surmised that there was a problem with the circuitry in the box. the dealer wanted $780.00 to replace it. so, i ordered the part myself and replaced it today.. still no wipers working and pushing down on the relay still does get the wipers to work.. i have swapped out the both wiper relays in the fuse box with new ones and still have the same problem. I was thinking it could possibly be a problem with the body control module or maybe (hopefully not the engine wiring harness). have you run across anything like this problem before? Any advice you can give to figure out this electrical mystery? Thanks!