I disconnected my rear view mirror from the roof wiring harness and next time I drove the car when I attempted to use the cruise control it failed and gave me a failsafe mode error.

I've had the cruise control fail like that before due to a vacuum leak that I had fixed. I'm wondering if its just a coincidence that the problem came back after I changed the mirror or if the cruise control uses one of the sensors in the mirror? Thanks,

 

You'll probably find that it's a coincidence, the suspicion normally lies with the brake pedal sensor when cruise control triggers fail safe mode. It's easy to remove and clean, or even replace. One nut in the brake pedal.

 

Remember, a 1998 has a vacuum controlled CC opposed to your car, although the brake switch still disconnect the operation.
I do agree though it is coincidence, the mirror wiring has noting to do with the CC operation.
First suspect should be vacuum again.

 

+1 for the brake switch. My cruise control was acting up, and I was getting a restricted performance message once in a while. I did not realize the two were related until I replaced the switch. It has been perfect ever since, with no messages.