We had this problem in the shop and it ended up being the BCM. I installed a new one but you might be able to plug in a used one without programming it.
Here is the wiring diagram and the BCM location so you can see how the system works.
Check out the diagrams (below).
Please let us know what you find. We are interested to see what it is.
Images (Click to enlarge)
Monday, January 10th, 2011 AT 7:34 PM