2004 GMC Envoy driver's side heated seat quit

If other functions such as window controls and power seat controls are normal, here are some possibilities;
1) Circuit Breaker #8 in rear fuse panel
2) Defective Driver Memory Seat Module (under driver's seat)
3) Break in data lines between Driver's Memory Seat Module and Drivers Door Module
4) Fuse 23 & 6 in rear fuse panel

There is only one fuse for the brake light, so if the passenger side is working, the fuse is OK. You most likely have a bad bulb, bad bulb socket or a wire issue. The circuit board that all of the rear light are mounted to also has had problems in the past.
BTW - the fuses for the rear light are under the rear seat.

bad earth on the bulb holder or the wiring, usually a good clean should sort it out.

Check Fuse #8 in the rear underseat fuse box, this fuse is also used for the power seats. The problem could also be the heater control module (located under driver's seat), a broken wire in the harness or loose connector under the seat. If it is not the fuse, then you will have to remove the driver's seat to check things out.

check the fuse for that seat it may have poped when it went forward.

There is one under the passenger seat driver side. Flip up the seat and lift off the cover.

I had the same issue with my 2004 XUV on the drivers side. Noticed that a "spot" was hooter than normal right before it quit. The seat heaters are basically a simple wire coil that is laid between the upholstery and the padding. The back and seat units are independent but operate off the same switch. If you are somewhat mechanically inclined, this is not a difficult fix, you just have to be patient and careful. Mine was a year ago and I think the replacement part from GM Parts Direct was less than 50 bucks. The dealer wanted over $300 to fix it. If you want me to walk you thru it let me know.

Check you connections both under the seat and at the switch. Many times corrosion will occur due to the amount of amperage that goes through the circuit.

Work for a GM dealership. the problem is the heated seat pad element is bad. Replacing the element is not easy but can be done at home.