Locking system still failing

I have a 83 300SD TurboDiesel and the entire locking systems has stopped working. Normally if you turned the key on the driver side door, all the locks woud unlock. Now everything remains locked. I can unlock and lock all four doors manually by just pushing the door buttons, but I cannot get the truck to unlock. I know the locking system is a vaccumm system, how can I go on trying to find the leak or what is causing this? I think the vaccumm pump is in the truck, but how can I get to it? One of my tail lights went out and I need to get the truck open to replace it ASAP.