One of the aux fans on my 95 E300 would only operate after tapping on it. The only sure fix was to replace it. They aren't that expensive. Surely a lot less than fixing the car afer it overheats. I bought my fan from Performance Products (before Fast Lane) and my tech charged one hour to
install it. One thing he discovered was a massive amount of dead bugs had piled up between the fans and the condenser, and there wasn't any way to notice it or clean out that area without having the fans out.