The wires for the rear brake light switch are a small sub harness connected by a plug to the main harness. This is the picture of that plug: This wiring is separate from the rear brake master cylinder float switch.
I believe these like the brake light switch are part of the main wire harness.
These float switches are part of the brake failure light on the correct instrument cluster for your bike. They are not used on BMW bikes after perhaps 1981. Bottom line, the instrument cluster you have now, will not work with the float system wiring but the plug in the back of your instrument cluster is wired for them.
This is how your plug and instrument cluster is supposed to be wired like. Anything else is incorrect.
I also don't have a spare proper instrument cluster I could send you. Mine are like the one you have now and are not correct for your bike.
Get a GOOD wiring schematic such as the one I have. Be of cheer in that if there is no obvious signs of cut, rewired or unplugged wires your wiring system is intact
Get the correct cluster.
Get the float parts for the rear master cylinder.
Check the wiring for cut or unplugged wires and determine if they are part of the problem.
Don't loose hope of fixing things, I have seen a whole lot worse messes by previous owners strange behaviors. St.
'78 R100/7 '69 R69S '52 R25/2
Looking at the diagram and picture Max has, I don't see the float assembly. I also don't see a part number for the rear float and level switch.
I am dead certain Shane's bike is supposed to have the float, and switch along with the sub harness 61 12 1243 616.
I would imagine when they made the change to the handle bar mounted front master cylinder they did away with the fluid level warning system altogether. There would be no need for it as there is no longer a bulb in the instrument cluster to light.
What ever year they made the change, I will bet 81, there was the corresponding changes in wiring as well as the change in the instrument cluster.
Once Shane gets the instrument cluster sorted with the proper one, he will have to start looking for the missing float, and the wiring to go with it. Or, he will just leave the fluid level circuit out all together.
I think with great certainty his problem lies more in the instrument cluster main plug for his bike is wired for the the lights he is supposed to have not the ones he has. I would imagine, the pins are close from the old year to the newer but not 100%
Lord knows how I dislike fools (not Shane) who take perfectly good vehicles, tear them apart for whatever reason then cobble them back together using bits and pieces that don't belong.
Of course, LOL, this means I don't agree with the new crop of cafe bike or custom bike shops out there, I have mixed feelings with them. I see nothing wrong in someone who wants to make a custom bike as long as they do a proper job of it.
Glenn at the Beemer Barn has a bike that was hacked up and "customized" to the point where it quit working properly. The previous owner who did the hack job unloaded it on a new guy who wants the bike to ride. The jury is still out on when the bike will be on the road if ever, so much was changed by the hack owner. Glenn is trying hard to keep his price to bring it back to running condition to a minimum but time will tell.
In Shane's case, who knows how much of a hack job the previous owner did on the bike. It would be nice to teleport over to AU to see what is on hand and to give a hand. For now, we are stuck with the forum and pictures. St.