For anyone interested, I have a Schroth Quick Fit Pro 4 point harness for the driver's side which was originally made for the Mini but also works for the E90/92 as well. This was before they developed the same model for the M3. Mine has black straps with black and yellow pads. These are designed to be DOT compliant (ie. you/EMS personel push one button and you are released). It latches into the seat belt receiver in the rear seat and the child safety seat support. A link is included below. It sells new for $400. $225 shipped and it is yours. Here is the link
https://www.schrothracing.com/products/quickfit