The BMW button may have 3 pins but if you want an alternative then, you only need a 2 pin momentary (non-latching) button, and connect like so.

----Pin32 from ULF (the pair wire)----][-button--/ --][---------Gnd

Thats all you need,

At a guess I'd say the OEM wires are something like

Switched +12v (for button illumination)
Ground.
Pair.

The 18 ELO plug also has a Direct +12v

HTH

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Sept 03, M3, Silver Grey. Retro-fits: Modded Mk4 DVD Drive, OEM Bluetooth, VR Control, Rainbow Speaker Upgrade, Sony SOT Amp, HK Sub, CCFL, Intravee II, NightMode.