I have a 98 gmc jimmy 4wd drive I have been having problems with, you can be driving it and it would just randomly shut off while driving it down the road or at a redlight,one time it did it, the service engine light come on, so I took it to autozone, they said it was the ecm coil,so I replaced it, drove it a hile,it would still shut off, no check engine light, I looked a the fusebox under the hood and the small ecm fuse was melted,not blowed but melted, so I pulled it out snd put another one in it, run fine for a while, then it would shut off still, thinking that it might have ashort in the fusebox, I bought one for it, it still shuts off, and I hear this clicking sound in the dash off and on, and sometimes when I turn ac on it would shut off, and sometimes it would would be cold and sometimes not, I even bypassed the heater core because when you turned ac on, I could see the clutch kick on but it would still hot air out of the vents, so when I bypassed it, the ac was blowing cold, and now the clutch kicks in when it wants, would there be a short in the ac/heat switch that would cause it to shut off, because I have not been using the ac and it hasn't shut off in the past few days,but even when I have the ac off I still hear this clicking sound in the dash,and when I take the hot wire off the battery and put it back on, it sparks big

 

The clicking sound in the dash might be the HVAC temp door motor running but due to a broken plastic gear not able to actually move the temp door so it just keeps running.

Empty the glove box of contents, then where the glove door latches, lightly pull on the latch which lets the glove box door drop down. Then you have access to the temp door motor. Probably best to do a search on the forum and find the pictures of this area than me trying to describe where to look. But listen to see if the clicking is coming from this area.
Could also be a vacuum leak at the HVAC control. Common problem.

As far as the melted fuse; this is going to require some troubleshooting to find the intermitted short that caused this. Probably best to get a service manual or at least schematics to be able to follow the wiring. Good luck on this one.