Still restoring my lovely 1991 K100LT, but not back on the road yet.
I'm looking to find out how the radio remote works on a 1991 K100LT, specifically what the little LED does.
My original BMW cassette play ist kaput, so I am replacing it with a newer Pioneer radio (that is equipped for "wired remote").
I believe that I can make an interface to connect the original K100LT radio remote to the newer Pioneer, so I can operate volume and two other functions, eg scan or preset up or down. 
The interface electronics are relatively simple, just a few specific resistors etc. I can post details when I get it working if anyone is interested.
So on the original LT radio remote,
does the LED light up as soon as the radio is switched on, or
does it light up when a remote button is pressed, eg Vol+ etc
Or if it does something else could someone video what it does perhaps?

Thanks in advance.
PS if anyone wants the original BMW cassette radio for parts let me know. I am in Australia

If I recall correctly the LED lights up when you press a button on the main radio to allow remote control.   I think it is the volumn control button.  Otherwise no functions on the remote work.

The remote is a fairly simple device.   Again if I recall correctly,  there is one common wire for the LED and all of the switches in it.  I don't recall any resistors, transistor, etc.  I needed to know about the remote so that I could wire a R1150RT remote to the K100LT BMW radio that I put in my R1150RT.

The Pioneer head unit wired remote uses a 1/8" plug (like earbuds) whereas the OEM handlebar switch uses a different circuit(wire) for each function. They operate VERY differently.  I suppose you MIGHT be able to use the OEM switch with the Pioneer head unit but you'd need to design and build rather complicated electronic interface.
 
Another option is to mount the Pioneer CD-MR70 (or 80) Marine Waterproof Remote to the handlebars.  It takes a little work but isn't too hard. That's what I've done.
 
http://www.kbikeparts.com/classickbikes.com/ckb.tech/0.ckb.tech.files/mvh-7300/pagename.htm

Thanks for the two replies. 
From my research, the "ear buds" connector uses a "ladder" of resistors, and depending on which resistor is switched through to the ear bud plug (by the switches in the original handlebar remote) the radio will respond in a particular way. It can be difficult to find the published resistor values for each function as it varies from brand to brand of head unit.  So I chose Pioneer because I found a table of resistor values for Pioneer.
The original remote has 4 switches (Vol+, Vol-, Scan up and Scan down) but by inserting the right resistor between the remote and the earbud connector you can get the radio to do those 4 things. Or, you can insert a different value resistor and that switch will do something different. So I'm setting mine up (hopefully) to do Vol+, Vol- Scan up and AM/FM select (as I sometimes like to listen to AM radio).
I probably should have opted for the marine waterproof remote as you suggest, as the "research" and building the thing has taken hours and I'm not finished yet!
Thanks again duck and kioolt for suggestions.

Interesting. I had no idea it worked by variable resistance.  Here's a pinout I have for the OEM remote.

Soooo, after quite a few weeks I got my pawn shop $9 Pioneer head unit installed and the K100LT remote working. I used the diagram from Duck (above) because the coloured wires from the remote switches on the handle bar are different at the radio end (there's a DIN 8 pin connector and the pins on that plug and that socket have different coloured wires attached - most confusing!!)
The remote switches connect via that 3.5mm "ear buds" plug and the head unit reads which resistor is being switched in - if that makes sense - and acts accordingly. I chose resistors that will make Vol+, Vol-, Scan up, and Select Band. You can make the four handle bar switches do whatever by choosing the appropriate resistor to solder in.
Thanks again for the suggestions etc

DiverPete wrote:Soooo, after quite a few weeks I got my pawn shop $9 Pioneer head unit installed and the K100LT remote working. I used the diagram from Duck (above) because the coloured wires from the remote switches on the handle bar are different at the radio end (there's a DIN 8 pin connector and the pins on that plug and that socket have different coloured wires attached - most confusing!!)
The remote switches connect via that 3.5mm "ear buds" plug and the head unit reads which resistor is being switched in - if that makes sense - and acts accordingly. I chose resistors that will make Vol+, Vol-, Scan up, and Select Band. You can make the four handle bar switches do whatever by choosing the appropriate resistor to solder in.
Thanks again for the suggestions etc

I used the original radio and handle bar remote but fitted a Dab wireless (apart from power supply) car radio adapter. you just tune it in to the head unit like you would a radio station. I did this because I can have dab radio and bluetooth which links to my phone and MP3 I did fit an extra aerial but works well and is removable as i wanted to keep the original set up. Something like this 
https://www.aldi.co.uk/dab%2B-%26-fm-receiver/p/098154267771500?gclid=EAIaIQobChMI0aG05NjT6QIVGLLtCh0q7wecEAQYASABEgLnW_D_BwE&gclsrc=aw.ds    

I have very poor wire`ing skills! so just an alternative  K.

Thanks for that. I like DAB but didnt think I'd be able to have it. I'll look into that device here in Australia

Just to give you an idea of my set up. I have got power from the Aux port on the dash pad wired into it but fitted an on and off switch so i don`t drain the battery. It also has a usb port for charging and will take a mini SD card with music but I just use the bluetooth and Dab. I put a small freezer bag over it if its raining just in case but where its placed it does not seem to get wet anyway! (probably not a problem for you guys in the land of OZ!)

Seems a bit bright but you can adjust the settings to a night mode, the handle bar controls all work fine but I just use the volume and select the Dab stations using the "M" wheel on the Dab adapter. Hope that helps K.