Well, I thought I would give this a go and it's more difficult than I thought. I bought some used rear multi-function indicators on eBay and 2 repair leads from BMW. The repair leads meant I could add a small custom harness between the bike's main harness and the rear light. That way I figured I could tap into the tail and brake light feeds without having to damage the main harness.
I got the new indicators functioning fine, no problems. The problems started with the tail and brake lights. There are 3 wires going to the tail light which I figured were earth, tail and brake. Wrong! They are earth, number plate light and one wire for the tail/brake. The tail/brake wire sends about 4V to activate the tail light and then increases to 12V when you use the brakes. The multifunction indicators also have a single wire feed that works the same way.
I got all this connected and then discovered that the Canbus detects a fault with three tail lights, throws a hissy fit, turns the circuit off and displays an error message on the dash. And you can't fix this with a relay due to the 4V/12V supply. The only way to fix it would be to reprogram the canbus so it allows a higher drain without sensing an error.
For now I have the new indicators working but without the tail/brake light function. At least it's all reversible, so no harm done.
Does anyone here know whether the canbus controller be reprogrammed to raise the current limit? I am guessing the answer is 'no', but I thought I would ask.
I am feeling very humble today.