BMW like to do special stuff with the canbus
I retrofitted the aux loom off a later bike and once I sussed the set up of the 4 pin connector under the tank thought I'd got it working but then ended up with the green led on constantly... and had to rewire
they have switched earths and live that are only live once the alternator is outputting stable current - all of which play with how an old boy thought electrics worked. Maybe worth checking part numbers between an old K25 bike and the later air cooled bikes.
the original loom for axu lights on 2006 to 2007 bike comes with a relay as part of the loom and does stuff they never needed to do... the 2008 bikes went normal using a different loom
pin outs for the 2006 to 2007 hex head bike's 4 pin connector under the tank (above the ZFE is actually)
Pin 1 Red Green - Perm +12v
Pin 2 Purple - Neg when engine running
Pin 3 Brown Purple - Ignition Switched +12v
Pin 4 Brown - Perm Earth
so using a 2008 loom I needed to cut the red and add in a relay.... I didn't document how the switch works and can't remember now but its very odd with 3 pins switching earths if I remember correctly
as for the Nav connector I found some more fun from the can bus here....
one pin constant earth (for want of knowing I'll call that pin 1)
the pin at the other end (pin 3) is 14.4 v when the alternator is stable
leaving pin 2 being a switched earth once the alternator is producing stable output but with 2v missing so you get 12V not 14V which is some special earth idea
key out of the bike the power drains away over time a bit like a capacitor leaking down, all very weird