Have a look also for x3 & 1 series (E82, E83) Lear amps., They are exactly the same as the z4 (E85) one but with a mounting hole in a different place, no biggy to mod I'd expect (not that I've done it myself).
https://www.ebay.co.uk/itm/BMW-X3-E83-PL2-HI-FI-SYSTEM-AMP-LEAR-AMPLIFIER-6990098-/113082093838