Glad you got it fixed.
When I first read the symptoms, the unusal way the coolant temperature light diode is set up came to mind. Here is an email I sent to someone a few years ago about that -
"You'll have to excuse the strangeness of the diagram, I removed all the unrelated parts to clarify the engine temperature idiot lamp.
The lamp is in the lower right corner and is fed by wire 44 from circuit breaker CB-6. It finds a path to ground through wire 55A, 55, and 55B on the other side of the firewall bulkhead connector. This then goes through the engine temperature switch to ground. The switch is normally open and closes above 225°F to illuminate the light.
The "Copper stud" is the battery side of the diode D1. Wire 55A connects to the anode and 55C to the cathode. (They think of everything...) The engine temp lamp illuminates when you press the engine start button as it finds a path to ground through the switch. (46A or 46F) The diode is there to ensure that if the engine lamp should illuminate while the truck is running, it doesn't also engage the starter via the start button.
A few days ago, I grounded wire 55B to the engine to see if the lamp would illuminate while the truck was running and I think I heard the starter engage!
I am going to test the diode today and see if it is working correctly. If it isn't, I may just ditch that setup figure out another way to provide a test function for the light. I mentioned that I removed a 250°F switch from the truck and have since replaced it with the correct 225°F switch. I don't think my truck has ever gotten that hot while I've had it, so I don't think I've inadvertently engaged the starter."