Well, my attempt proved reasonably successful. I had a pair of spare mirror caps in black that I'd removed from when I purchased the folding mirrors, so I used those as my test subject.
I popped a 18mm hole in the side of the mirror cap using the hole-cutter for the parking sensors, then used
these ¾" LED marker lights
and simply spliced the wires into two BSM wires that lead to the mirror glass; got them at Aliexpress, but I'm sure Amazon has something similar -
this looks like them.
After I figured out the two-wire wiring (pro tip: just because your wires are white and black, DO NOT assume that the black wire is ground
), it actually turned out quite well.
(Frame rate of the camera and the LEDs not playing nice - in real life, both LEDs fade in/out smoothly.)