Here is a copy of a post I send to people that want to do the heated mirror mod. This is for 05-06 trucks but should be the same for others. (I actually cut the wire from the fuse panel and attached the end that goes into the wiring harness to the number one upfitter. I then took the other end coming out of the fuse box and attached a female bullet connector. This gives me another fused ignition on power source. It is easiest to remove the four bolts that hold in the fuse panel and pull it forward. The wire you want is on the middle connector, on the back of the fuse panel. It is on the left side of the connector, second from the bottom. It is green with a purple strip. You can double check by hooking up a test light or meter and turning the ignition on. It will then show hot(12v). Next remove the fuse for the mirror heat and retest. If it is cold you know you have the right one. Cut the wire about an 1 1/2 from the fuse panel and strip both ends. Put a connector(bullet or butt) on the end coming from the fuse panel and attach the other end to the upfitter wire of your choice.)