The following is from the original thread, describing the problem and the solution:
My cruise control won't work. Here's what's happening: I turn it on, wait a couple of seconds, then push the set button and it turns off when I do. Anybody had this same problem or have any insight?
If it acts up again, you may need to do what I just did to mine last week. My symptoms were EXACTLY like yours: I would turn the cruise on, the little red light was on, but as soon as I hit the "Set" button, the red light went out.
I traced the problem to a little relay that is built into the circuit board that is part of the complete stalk assembly, about $400 from a dealer if I recall! The contacts on the relay get pitted and corroded over time, and resistance builds up. Then, as soon as you put the complete load across the contacts when you try to set the speed, the voltage drops completely across the bad contacts. My solution: I went to Radio Shack and bought a small 12 volt SPDT relay. I removed the stock relay, without removing the complete assembly from the steering column, which was quite a trick in itself. Then, I just soldered 4 small wires on to where the original coil, Common, and NO relay contacts were, and ran them down behind the dash to the new relay. If you're not sure which wire is which, just remember: the tiny blue and white wires are the 2 that go down to the connector harness that get shorted together when the relay is energized, so you want one of them to the Common, and one to the Normally Open contact. The other two relay contacts are the 12-volt coil contacts. Have someone familiar with relays help you out. Good luck, mine worked perfect the whole 1100 miles back to Florida!
Another tip: remove the cap from the top of the original relay, and you will easily be able to see which of the 4 legs on the back are which. You can plainly see which ones lead to the contacts, and which ones lead to the coil of the relay. The hardest part is turning the steering wheel to try to get to all of the screws that hold the assembly on to the steering column. I managed to do the whole job without pulling the steering wheel, or even dropping the column down from the dash.
Link to original thread
edit: If you opt to remove the steering wheel, here's a thread that includes info on that.