I am still on the fence but the TeslaOffer one looks solid. The connector in the 2021 m3 is gone though so he is recommending a posi-tap. It can be wired without the posi-tap but that connection is coming later and will need to go up through the door so they are recommending the posi-tap.
I am reluctant to do the posi-tap as its not something I have familiarity with...but would love to see others who have confidence report on it...or maybe find someone who is more comfortable to do it for me!
Posi-tap is very easy to use. I use T-tap because it's cheaper. Too bad Tesla keeps removing hardware on the car but then again, the 2021 (re-fresh) has a few added like wireless charger and powered trunk, new headlight and heat pump and double pane front windows.