Thank you Radar, I suppose a bit of clarity on my setup would help! (Sorry 'bout that)
From the radio I've got four channels out. This used to run into a four channel amp, which has now been replaced by the MS-8 and an MB Quart four channel amp. The MS-8 also feeds another MB Quart that powers the sub.
Since the BMW harness was already cut for the old amp, I simply re-made those splices and ran them into the MS-8 hi level input. Ch 1 = LF, Ch 2 = RF, Ch 3 = LR, Ch 4 = RR, Ch 5/6 = Sub.
During the MS-8 xover setup, the fronts are seen (i.e. I can set the type, one way in this case as they have their own crossover) but when I get to the rear the MS-8 reports (none), and this is the only option I have.
After I've completed the setup the fronts are working, but I've got nothing from the rears.
And yes, I've individually taped all the unused inputs!