The wiper board is on the upper (center) part of the firewall. You will see the wiper box and it is inside that box. The board is removable and I replaced mine and the wipers work well now. I hade the same problem, when raining outside they would not work, but if hot and not damp they worked fine. You might take the board out and try to clean it and reseat it back in. Try using the search forum on this site, you will find a lot of answer to your problem and a lot of problem you don’t have yet. For parts try eBay or
www.gmpartsdirect.com. Dave