On Wednesday, December 1st, I just purchased my first K100RS from a gentleman in Lawrence Kansas. I'm down in Arkansas. I joined the forum yesterday. The bike is not currently in full running order. The first issue is an intermittent short in the Kill switch. The previous owner was upfront about the bike's condition and the price reflected its issues. I was able to ride it a couple of blocks and she starts fine, shifts, brakes fairly well, but she dies often due to a short. When it dies, it starts right back up. Previous owner thought he had the switch sorted out awhile back but I guess not. After a quick lookover, she could use the following:

1. New tires (old ones are 2012)
2. Typical Maintenance Items (fluids, filters)
3. ABS Dash Switch
4. Hazard Dash Switch
5. Fuel Gauge (mine very foggy)
6. Instrument House Cover (mine is cracked)
7. Odometer Refresh
8. Battery Cover Grommets
9. Coolant Reservior (mine is stained/cloudy/nasty)
10. Fuse Box Cover

I gave her a good wash yesterday and took a video of the KILL switch problem...available on YouTube: KILL SWITCH PROBLEM VIDEO
Last night, I hit the internet/eBay and started buying parts to bring her back to life. 

As I refresh her, I'll try to post pics. I'm no professional restorer. I'm just wanting to get her running reliably/safely and looking good.

Here she is: 

You're on the right track

You'll need a replacement fuel gauge because the anti-fog layer has peeled off the backof the face.

Coolant reservoir: to me that sounds like you're actually looking at the rear brake fluid reservoir as the coolant overflow bottle is black and under the r/h side panel. If you are talking about the brake reservoir, then two things:

1. you need to take the battery tray off to unbolt the damn thing - a not-their-finest-hour BMW moment because the battery tray may not want to come off without sheering the anti-vibration mounts
2. get an aftermarket replacement and fabricate a bracket to mount it from between the r/h footrest hanger and the gearbox. Lots easier if it needs to come off in the future.

OTOH you have the later 4-valve model so I may not know what I'm talking about as usual

Welcome!  That's a pretty nice looking bike.  It's a sister to the one in my avatar which has had some customizing done to it.

Before you go and replace the coolant overflow reservoir, do a good flush with white vinegar followed by a distilled water rinse.  You may also have to do the rest of the cooling system as well.  A little bit of rubbing with some toothpaste on the sightglass will make it easier to see the level.  Unless it's leaking, there's no real reason to replace it.

The tape on the speedometer glass tells me you have an ABS fault.  Once you've gone through the rest of the bike and have it running well you can address that. I suspect that if the bike sat for a long time it may be fairly easy to correct.

Definitely get a can of Deoxit D5 electronics cleaner.  There are a lot of connectors and switches on your bike, and they probably have dirt and corrosion issues.  A shot of Deoxit everywhere a connection is made will go a long way to making your bike as reliable as a new one.

Thank you for the tips about the contact cleaner and coolant reservoir. I will give those a try today or tomorrow. :-)

You have a great pocket rocket there.

Brought one back from 12 years storage last summer.

The ABS might be the big issue. Use a power bleeder to suck the fluid through the system. If something is clogging either master cylinder or ABS box, you want to pull it out, not push. Better chance of not tearing a seal.

If you can't get the ABS fault light to clear, remember it will still haul you down quite well...just no ABS.

Did the bike come with the pearl white bags? Great touring machine

Concentrate on getting the bike detailed and running well before screwing with the ABS.  From my experience(five bikes with ABS1), the system is pretty robust, the problem is probably a sensor gap, dirty connector, or low battery voltage.  Get the bike running, make sure the battery has a good charge, do a diagnostic check followed by a reset and then go for a ride.  With a bike that's been idle for a while, that may just do the trick.

Hi OP posted of problems:
  5. Fuel Gauge (mine very foggy)
to which  Dai posted:
  You'll need a replacement fuel gauge because the anti-fog layer has peeled off the backof the face.

I could be wrong but I couldn't see any fuel gauge in the two photos except the usual low fuel light in the instrument pod.
But if it's just a matter of the lens being foggy then a regular tungsten filament light bulb shining down
on the lens has been enough to clear out the fog/condensation for me in the past. Main issue with
that fix is making sure the tungsten light bulb isn't close enough or gets the instrument pod hot enough
to melt. Want enough heat to drive out any condensation but not so much the pod plastic melts.

I can see the break in the instrument pod lens just above the low fuel light. If no plastic
has been lost I've had luck with CA/crazy glue being applied and holding the lens together.
Doesn't look as good as original but seems to be water tight until you find a replacement.

Darn...finding more things wrong with the bike as I go through it. Today, I replaced the faulty right COMBO switch, HAZARD switch and ABS switch. Those changes went fine. 

I also took the time to drain/remove the tank, drain the coolant and add vinegar to help clean out any gunk. When I started the bike to help circulate the vinegar, I noticed the temperature gauge never moved. I also noticed the fuel gauge constantly indicates between 1/2 to 3/4 even though it has very little fuel in her. The bike also refused to start after adding more fuel. I can no longer hear the fuel pump. So, this adds:

Problem with water temperature sensor/indicating circuit.
Stuck fuel gauge (It was getting replaced anyway.)
Possible fuel pump/filter issue.

Darn the bad luck.

How about to take off some parts, and clean EVERY connector you can find, also the earthlines

Absolutely....standard practice.