This is the second thread like this that I've been involved in. Today after work I decided I would tear into my gauge package and see what the heck was really going on.
20 minutes later we have this:
Here is the gauge package wiring diagram for the '87:
So what did I do and how did I figure it out so quickly? Well, first of all, I took nothing for granted. In the wiring diagram I saw the 20A fuse for the courtesy lamps, clock, and dome light. That was the first thing I checked. Turns out the PO put a 10A fuse in. Of course it was blown.
Second, I took apart the gauge cluster and tested pins 3 & 7 to verify 12VDC at the plug. Good to go. Checked it at the back of the clock. No good.
Third, I noticed that the printed sheet looked pretty nasty right by the clock. In fact it was separating. I gently peeled it apart and put some dielectric grease on the screw terminals at the clock as well as squirted some under the printed sheet area that looked questionable. Reconnected the wiring harness and CHA CHING!! the clock started working!!!
Moral of my story: Don't jump to troubleshooting step E before performing steps A,B,C and D.