I am new to this forum, but I also drive an '85 500 SEL. Mine has 254k miles on the odo. I respect you very much for opting to keep yours stock. Mine is, and I can't tell you how much that means in the long run. My most sincere apologies if I go on telling you stuff you already know. I'm trying to break it down for everyone else's benefit.
As a general statement, this particular model and year has certain advantages over a 1986 or newer, second-generation W126. For starters, the systems were very refined and streamlined. There were few to none of the problems typical for first-model-year cars.
The M117 engines are really, really well engineered for a car from 1985. The Bosch Jet-Tronic F.I. system is really sturdy and durable. The main things that need to have attention given to them in this engine series are the following:
* Timing Chain replacement -- Very Important!!! I change mine every 70k miles now. These are "interference" engines and you will have to rebuild the cylinder heads if the chains slack or break ($$$$). The previous owner of mine broke the factory-installed chains at 171k miles and had to rebuild.
* Filter replacement -- I use the Severe duty cycle from the manual and new BOSCH filters.
* Tune-up with new plugs every 20k miles. I use Champion Single Platinums for cost-efficiency.
* New Wires when needed. I'm on my second set in 50k miles.
* My MB Mechanic said there is a 3mm allen-wrench slot in the air cleaner assembly that can be used as an adjustment for correcting rough idle. I haven't needed to do this myself. I can ask him more about this if you need the info.
* Belts every 50k miles.
This series of engines has been known to last for a very, very long time if maintained well. You will save $$$$ in the long run by putting $ into it every little while. I have personally seen a W126 with the 5.0L going strong at 420k miles, on the original motor.
No, I haven't seen any W126 5.0's with an aftermarket dual exhaust that looked good or sounded good. Keep the Euro look by going with something like this: 500SEL AMG Exhaust
One big thing to consider replacing would be the shocks and some brake components. These parts are, of course, designed to wear out and need replacing. I chose the OE Spectrum series from MONROE for my shocks. I replaced the calipers and the master cylinder with Centric brand parts I got on RockAuto.com.
I have run into similar issues as you have with the climate control, but not the radio. I am still having fun with the electrical systems, as the previous owner didn't hesitate to splice, chop and solder 'til kingdom come....
In my case, I had two separate issues on the Climate Control (mine is the thermostat-based "little wheel" automatic climate control). The first problem to solve was getting power to the dash (control) unit, the second was getting power to the blower unit in an orderly way.
Now comes the troubleshooting guide for the electrical. You can use this for all of your electrical troubleshooting on the various systems.
If you aren't handy with electrical systems, are really afraid of getting shocked or don't have time for all of this, print this post off and take your car into a decent local mechanic with good electrical know-how.
My personal recommendation is to avoid the national branded places like pep boys and especially the car audio shops, because our cars are special. For instance, a young mechanic or car audio guy with minimal training and experience is likely to make things worse or, at best, just waste your time.
For a little basic information, your fuse panel is the best place to start, as it is the very easiest to access. I agree that it could be a relay, so let's start with the fuse panel to narrow it down to which relay it is.
1. #1 or #2 Phillips head screwdriver, plastic handle (usually less than $1.00 at a hardware store).
2. One or two packs of GBC 8A fuses -- white (about $3-$4 each at most places).
3. Digital multimeter (cheap at HF, expensive most other places)
4. Flashlight (preferably a small one or a head-mounted style)
Start by taking a look at fuse #10 (White), which according to the info card is the circuit for a host of things, including your reverse lights, your climate control dash unit and the little temperature display that's right above the steering column on the dash. As a side note, try this test: set your parking brake and put the trans shifter into reverse with the key in the "on" position and the engine off, see if your reverse lights come on.
You said this fuse (?) was blowing as soon as you gave it power -- if this is happening, you must have a short or a overload. Start out by checking the terminals under the fuse slot for #10. Remove the two Phillips-head screws securing the fuse panel to the plastic fuse-box case and lift the panel up and forward to pull it out. Do not hesitate to pull firmly, you won't hurt anything. Now, take a light and look closely at the wires hooked up to the screw terminals on #10. In my case, one of the screws vibrated loose over time and wasn't making a good connection. Make sure that each wire that is hooked up has the little factory sticker on it that says "10" (this is about 1 inch away from the end of the wire) or whatever number of fuse you're working with. If it doesn't please reply to this post immediately and put up a photo, please.
If all your wires are in the right places on the fuse terminals, we have eliminated the possibility of a problem in the fuse panel for this circuit. You can close the panel up and turn the ignition on to see if you are still having the problem, and if you are, we will go right back into the panel to test each wire individually.
If you have a Multimeter
handy, then check the power coming in on the black power supply wire. You should get 13.8 volts. If this is checks out OK, we should look at each of the load wires individually, which requires connecting ONE OF THEM AT A TIME to the load side of the fuse terminal. You should have a pack or two of white (8 amp) GBC fuses handy for this troubleshooting, in case you blow a few of them. Fuses - 5 pack
Connect each load wire one at a time until you find the one that is blowing the fuse. If none of them do it, then start checking them in pairs. Once you find the one that is doing it, note what color it is and then cover the end of it up with electrician's tape. Reconnect the remaining wires and close the panel up for now. Go down the list of systems on the info card, testing each one to see which one that wire belongs to. Now we can find the appropriate relay to replace.
If replacing the relay(s) doesn't do it, we need to look at two things: 1) the wire runs to the device/system. 2) the connections to the system (i.e. the terminals on the back of the climate control unit in the dash.
This is all the time I have for now... I will try to follow-up as soon as possible. Please don't hesitate to reply to this post or send me a Private Message with any thoughts / observations / questions / grumblings....
Hope this helps!