One guy in Europe used a bit one connected to the OEM HU thru the high level inputs: Pics of my install - PDX 5 Audison Bit 1 - BMW 1 Series Coupe Forum / 1 Series Convertible Forum (M1 / tii / 135i / 128i / Coupe / Cabrio / Hatchback) (BMW E82 E88 128i 130i 135i)
I think that you could do a straight connection as well thru the low level inputs using RCA plugs.