Audio upgrade T-harnesses for M37, 56 and Q70.
Have not tried, do not know anything about them other than what the website lists...
Will probably try the Amp the Door harness first.
https://www.owc-upgrades.com/product-pa ... num-series
https://www.owc-upgrades.com/product-pa ... s-red-line
https://www.owc-upgrades.com/vehicle-upgrades?page=6
