windows problem on my 91 240

My '91 240 has developed an annoying problem with the electric windows. That being that they have stopped working most of the time. I've not had time (working 80 hr weeks for a change) to really look at them. What is happening is that they only work intermittently and the time they seem to work is when it has rained. Today I had to run an errand (and it's raining hard here) and without thinking I rolled the drivers side window down and then turned the car off. When I jumped back into the car the window was down and I had a wet ride home. I would think it was just the drivers side switch but what ever the problem is is affecting all the windows. Ideas? And thanks in advance.