Trying to Diagnose a Solid Theft Light

Hey Guys!
I'm the original owner of a 1999 Mustang GT and I'm having an issue with my theft light.

Seemingly every other day I come out to my driveway and see that the THEFT light in my dashboard is lit up solidly. Even when the car is off. The car starts and runs perfectly every time however, I can't seem to find a way to make that dang light go away for good. Sometimes I leave it alone for a few hours and it goes away by itself. It seems to be totally random.

Disconnecting the battery for an hour makes it go away, but only temporarily.

According to my research this may be an issue with either the key itself or with the transceiver (immobilizer module) within the steering column.

I'm going to the dealership on Friday to replace the transceiver and get new keys, but I was wondering if anybody else has figured out a solution to this problem.
There are a couple of forums about it scattered throughout the internet, but none of them seem to give a solid answer.

Alrighty!

So, dealer turned out not to be super helpful. Let me explain

Of course, as I'm driving to the dealership the theft light went away. So I went and just tried to get some info on the problem. They said it could be the transceiver inside of the steering column. Unfortunately, Ford does not still produce these so you will have to source them elsewhere

For SN95 Mustangs, the PATS transceiver part number is F8SB-15607-AC


I got new keys and key fobs and so far the light hasn't come back. If it does I will throw in the transceiver and see if that fixes it, but for right now it looks like the new key and fob may have done the trick. I'm hopeful that the new key and fob will be enough to do the trick

No dice. Light is back. Going to try replacing the battery. If that doesn't work, I'll go ahead and install the transceiver

Okay. Here's where we are at.

I think the source of the problem is actually with the Power Relay Module. I think the wire going to the PCM is intermittent. I'm gonna try changing that as well as installing a new ignition switch. Unfortunately the module is on backorder so I won't have a definitive answer on this for another few weeks

The module is backordered. I'll report back after the install... Won't be for a few weeks probably.

Sorry for the delay. Just installed the new Body Control Module, as well as a new ignition switch. So far there is no light, but I'd give it another week just to be sure.

Hey y'all so far so good. I think it's safe to say the solution to my problem was to replace the Module.

It's about $150 on Cj Pony Parts
Part Number RCM1

Relay Control Module Remanufactured Mustang 5.0L 1994-1995/ 4.6L 1996-2000​

Also, if you have a hard time finding it, it's in your passenger side front fender.