Switches and relays that bring in the window motors carbon up over time. If it's easy to replace, get a new set of switches or have them cleaned out and tested. Secondly, the Body Control Module contains many relays that need replacing or cutting open and cleaning. They will also likely be dry-jointed and need a re-work. You can clean relay contacts by pinching a thin business card in them and pulling it through until you get no more black trace on the card.