GM had several circuit boards (used on many of their vehicles like pick-ups and vans) that had cold solder connections. I fixed them once, but it seems that the boards had some type of contamination. This is just me talking though; I work for a circuit board manufacturer so I’m familiar with them. The easiest thing to do is to replace the circuit board, which controls the wipers. They cost $35 at Full Service Auto Parts and it takes only ten minutes to replace.