It it very easy to leave the heated mirror on when it is not needed, and then it will eventually burn out.
I added an LED indicator that will light when the mirror heater is ON. Here are a few notes.
I put the LED in a hole near the bottom of the small speaker that is just inboard of the driver mirror.
The whole door panel has to come off (section 70-13 in my book). You do NOT have to disassemble the mirror shell.
There is one Torx screw on the bottom of the speaker enclosure. Next to it is a snap-in connector, and another is near the top. Pry carefully. There is foam behind the speaker that tends to be sticky. You will want to trim some of it to make room for the LED and wires.
I found an old LED and put a 400 ohm resistor in series. (The long lead was the positive lead.) Test it with the battery before installing it. I think some LEDs have a built-in resistor and others MUST have a resistor in series. Ideally you will adjust the resistor to the best value, checking in both day and night conditions.
On my 2000 GTI, the positive side of the LED goes to the white wire that goes to the mirror assembly, and the negative side goes to the brown wire. The relevant wiring diagram in my book is No. 27.5 on P 97-216.
One more thing: BEFORE you remove the door panel, mark where you want the LED on the speaker housing. The panel covers quite a bit of the bottom of the housing, so you will need to drill the hole about 1/4 inch below the "grill" part of the speaker housing.