How to turn on heated mirrors - 1999 GMC Jimmy

Short to ground somewhere in the circuit

see if this helps you out.
How to fix heated mirror Google Search

1999 blazer mirror problems. First I would replace the mirror. You can get an after market right side/left side, heated on eBay very cheap. that's where I get mine. They work fine. But you say the mechanic never fix your other problem in the first place. So I wouldn't no where to start. What is the other problem? Im just taking a guess here. But check your connections under your door pannel. Make sure no wires are bare or crushed. Make sure if any, fuses are not bad. Make sure your heat system that runs the heat to your mirror's is operating proper. You might have a short somewhere or maybe its just time to replace the mirror. To me when you see an amber color to your mirror it's the same as a light bulb blowing out or the mirror is getting to hot and shorted out. Good Luck.

It's not a heater it's probably a accusatory that adjusts the mirrior

The replacement mirror that you buy at most auto supply stores are not to be used as a replacement for a heated mirror due to fact that the materials used to make the heated mirror are different. You will need to replace it with the same type of mirror from the dealer.

Could have a DDS an PDS .depends on what RPO code yours has >
Outside Mirror Description and Operation (with RPO DL3)
Or
Outside Mirror Description and Operation (without RPO DL3)
Without
Heated Outside Rearview Mirror Operation
The REAR DEFOG relay controls the operation of the heated outside rearview mirrors. When rear window defogging is requested, the REAR DEFOG relay is energized and B+ is applied to the supply voltage circuit of the mirror heater element, located in the mirror face. A constant ground is also applied to the mirror face. Current will pass through the mirror heater element, warming the mirror, so long as the rear window defogger is commanded ON.

With
Heated Outside Rearview Mirror
The DDS and PDS control the activation of the mirror heaters. Upon receiving a serial data message indicating a request for heated mirrors, the DDS and PDS, will apply voltage to the mirror heater element supply voltage circuit. The mirror heaters will remain active until a new message is received indicating rear defogger is to be deactivated.
The power mirror system consists of the following components:
• Driver Door Switch (DDS)--An electronic control module with integrated switches and requires programming when replaced
• Passenger Door Switch (PDS)--An electronic control module with integrated switches, requires programming when replaced
• Power Mirror Direction Switch--Integrated into DDS and is a four position momentary switch that provides a signal input to the DDS for up, down, left/extend and right/retract with a mirror switch activation
• Mirror Select Switch--Integrated into DDS and is a three position switch that allows the operator to select the left or right mirror and also a neutral/fold position
• Mirror Extend/Retract Switch--Integrated into DDS and is a two position switch that allows the operator to select the mirror extend or mirror retract position
• Left Outside Power Mirror--Contains both the vertical and horizontal mirror motors
• Right Outside Power Mirror--Contains both the vertical and horizontal mirror motors
• Body Control Module--An electronic control module that requires programming when replaced, serves as the power mode master and is the main message handler
• Horizontal and Vertical Position Sensors, if equipped with memory mirrors--Supplied a 5-volt and low reference by the DDS/PDS and provide a variable voltage signal to the DDS/PDS that is proportional to mirror position
• Folding Mirror Motor, in each of the outside mirrors, if equipped with power fold mirrors--Provides extend and retract movements of the mirror and is serviced as a complete mirror assembly
Your best bet mite be to take to a qualified repair shop , i really don't think you'll be able to do anything with it.
Do you have wiring diagrams so you know which wires to test ,do you know how to do electrical testing ?
The Driver Door Switch (DDS) and Passenger Door Switch (PDS) supply voltage to the outside rearview mirror heater elements upon receipt of a serial data message from the HVAC module indicating that the rear defrost system is active. The door switch supplies B+ through the driver/passenger mirror heating element supply voltage circuit when a request is seen. Ground is applied through the driver/passenger mirror heating low reference circuit.

They are suppose to be automatic on the cars I know about. When the temperature drops to freezing, the heaters start working.

If moisture (frost/fog/water after car wash) rapidly disappears from the mirrors when you turn on the rear defroster, then they are probably heated. They don't always label these things. My Chrysler Concorde lxi has heated mirrors, but there is nothing written on them to indicate this. I know they're heated because car ordered from factory with them. Hope this helps.

Heated mirrors have a swtch in the car. That operates the eliment that is stuck to the glass.This eliment heats the mirror when turned on.It is used to defrost or demist the mirror.A none heated mirror has no wires going to it and no eliment at the back off the glass.OK.

Heated mirrors usually have a symbol on them -- a small diagram showing squigley heat lines.

If you don't see this, it's probably not heated. But when you take the mirror off, if it's heated it'll have wires connected to the back.

Yes, you can replace a heated mirror with non-heated -- just leave the wires tucked out of the way when you install the new mirror.