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I have a plethora of little electrical gremlins in my car. I am wondering if there might be a common cause I'm not seeing here, so I'll just list the ones I remember for now and maybe one of you can see some connection (see what I did there?)

Water temp gauge goes to zero occasionally
Right front door reads as open occasionally (this leave the footwell and door lights on, draining the battery, and prevent the alarm from arming)
Alternator whine in the stereo system (aftermarket HU with stock BOSE)
Buzz in the stereo occasionally (constant pitch and amplitude, separate from revs, and overlaid the alternator whine that is always present)
Occasional battery drain (seems too much to be just the door lights left on for an hour)
Occasional low voltage reading in the dash ("low" being less than the normal 13.8V, probably around 13,2V)
Occasional Airbag light in the dash with a child in child seat (ISOFix) in the right rear seat
Climate control always blowing to the windscreen in auto. It's never ever (I've had the car for three months) done anything else. In manual I can make it blow anywhere.
Dimming rear mirror doesn't work (wing mirrors dim, but the rear view mirror never dims)

That's the ones I can think of right now. No OBDII fault codes, I don't have a proper VAG tool to read the other faults.

Hope someone could have a look and maybe find something in common here.

Door lights will be the lock unit in the drivers door. The lock contains a module that sensors if the door is open. They pack up. Try spraying some lube in there. Wd etc.

The noises from your stereo may be a faulty earth or a duff connection.

Try vacom for other issue to pinpoint the fault

Sounds like a bunch of separate electrical issues. Resolve them one by one. You'll need a Vagcom to diagnose the A/C unit.

Hello, temp gauge is either temp sender un engine bay or faulty thermostat or maybe both...

Check faults in engine ans instruments

Good hunting

Let me know how you get on with the alternator whine through the head unit mine does this too! Was thinking of checking all the wiring and putting a suppressor on if worst comes to worst !

I had the alternator whine in my a4, I put some ferrite suppressors on my headlight earth wires as we realised I only got the noise when the headlights were on. Apparently it's a common problem when people put the hid kits on their cars.. ( which is what I'd done!)

Got a CEL today, P0118 - Engine coolant temperature circuit high input. I assume this is related to my cooling fans sometimes running on full even on a cold engine.

What make of battery, age, amp hour etc does it have?

It's about two years old, but failing. It's installed by the dealership and has an Audi part number, so I assume it's the same as the stock battery.

The fans blowing on full even on cold engine, and the P0118 fault code should be related, right? Also, my cluster water temp gauge is simetimes showing zero. So, Which sensor do I replace?

Looking at this diagram I assume it could be either "7" or "11"? Maybe both?
http://www.jimellisaudiparts.com/showAs ... ct=5572776

Check all your earth's as it could be linked... a bad earth can cause all sorts of strange issues.. i dont think it will cure all as not all are related but its good place to start.
I'd replace the battery for a good one too!
As Shop says though you need to work through one by one.

Does anyone have a diagram of the electrics, earth points and/or anything other helpful with these issues?
I had a Toyota MR2 GTi a while back and a book with EVERY electrical thing in the car, with schematics, every wire colour and every connector in the entire car. That was invaluable for diagnosing a very tricky electrical issue in that car. Can something like that be found for the RS6?

I'd get a new battery, ideally a bosch. Is there an old tracker unit it in the car somewhere? They can cause a large drain. There's also the boot microswitch I think as well that can cause an issue.

The fuse box area can also let in water so that's a great source of potential problems.

I had an exide battery put on mine and the car was just weird until I changed it back to a Bosch.

I think the mahoosive service manual might have wiring diagrams.

Where can I get this massive service manual? I've been looking for that for a while.

Bentley manual or elsawin from eBay or similar.