Reverse lights on tow mirrors

If it's the small led in the mirror glass then it's just a light to help light up the door handle.

Lwiddis wrote:
“Task Lighting - Press repeatedly to cycle through the out-
side mirror lamp options: both lamps on, left lamp on, right lamp on, both lamps off.

These are forward lamps, and work great after putting in Park.

The backward mirror lamp turns on with the cargo lamp switch, when vehicle is in reverse. This also illuminates a hitch lamp.

Your friend probably used a diode on his 2012 to jump pins just downstream of the BCM so that the rear-view mirror lights come on when he puts the truck in reverse. I've done the same on my 2018 -- cargo light comes on in reverse too.

This thread discusses it for my model year, but I can't say whether this will work on a 2020: https://www.gm-trucks.com/forums/topic/207166-reverse-lights-in-towing-mirror/

On my 2019 if you press the switch next to the exhaust brake switch the lights will work with the reverse lights when backing up or the cargo light if you are not in drive.

“The internet” says they operate with the cargo light turned on.

I just bought a 2019 with these lights. I assume the 20’s are the same. Hit the switch for the rear cargo light and as long as you are not in drive the mirror lights will come on

“Task Lighting - Press repeatedly to cycle through the out-
side mirror lamp options: both lamps on, left lamp on, right lamp on, both lamps off.

I have those. On the headlight/parking light knob, it’s the button at the bottom. Only work if the engine is off. Both on, left on or right on. An additional puddle lamp to the front.