I just Googled ***8220;electric folding mirror retrofit e46***8221; and found these:
https://forum.e46fanatics.com/showthread.php?t=900070

https://youtu.be/4oJJMSwuo44

https://youtu.be/b6OSNpeYmMs

There were others...

 

You need the mirrors with the folding motor, some wiring, mirror control switch with fold button, and a so-called "mirror folding module". This module can be aftermarket; essentially a timer-based circuit that uses a trigger (ignition on, press on button, etc) to supply 12v to the 2 mirrors' motors. On 2nd press the polarity is reversed (fold/unfold).

 

two great posts above.

having done this about 14 yrs ago on a early non powered seat car and then a third time with a car with power seats. let me just say it’ll be a more involved install being manual. having been one of several early pioneers finding info on this, it’s not too difficult following the directions now that’s out.