1994 GMC Jimmy- car won't start when it rains

I have a 1994 GMC Jimmy. When it rains, I am unable to get my car started. With a hard rain, the drivers side floor will puddle. I am trying to find out if anyone has had this problem with their Jimmy or know if this is a common experience with this vehicle. I am guessing that there is someplace where the rain water is leaking and maybe leaking on something electrical. Once the rain has stop the car takes a day or two (assuming to dry out) and then it starts right up.
Lastly, I am trying to locate the computer on this vehicle, does anybody know where it is? Thanks for your help.
-cathy