Mercedes: 1998 sl500 MBZ Sport Pkg, dashboard lights, fuse holder

Question
Thank you, in advance, for your time.  I have a 1998 sl500 and I have been having problems with: right turn signal sticking "on" but no flash; no horn; high beam working on and off but not "flash" as when you pull the high beam stem back to flash someone; auto door locks would lock me in the car awhile ago;  my gas gauge is not registering properly (I filled to almost overflow and it registers 1/4 down on the dashboard gauge) the "back light" of the clock and instrument cluster seems weak and inconsistent across the various gauges and, on one occasion leaving the dealership service, ALL my dashboard lights and gauges didn't work at all.  ...service guy said they "went and bought a used wiring harness and installed for me"...and then the instruments worked but...still the problems listed up above.  The bezel behind the horn and at the cluster of signal/cruise stems was loose and just tightened.  fyi.  And I had a blue tooth stereo installed by a pro a while ago--no problems after--but he did install the microphone with wire around/near steering column culminating with microphone dead center on top of the steering column.  My new mechanic a) replaced my horns and said they had died b) says I need a new turn signal part (relay?) c) says I would need a new high beam switch to be able to flash....arrrgggg.  Could it be all these issues are related?  I thought they all might be related to a bad/poorly installed wiring harness?   Or could it be that everything is needing replacement all at once???  The odom,which works, is around 110,000  thank you for any help

Answer
VK,
I expect that you have multiple issues with the electrical system. Clean wiring connections and proper grounding is critical to proper system operation.
It is very difficult to pin down the culprit.
I would start with the battery terminals and leads. Clean suspect connections until they are bright. Aluminum connections in particular tend to corrode first.
Check your fuse holder clips ant the fuse ends. Clean all the terminals and fuse ends. If you have ceramic bullet fuses, replace any aluminum strip pieces with copper strip units. Polish the copper before installing into the polished clips.
Certainly if the wiring harness was used, then you have potential for corroded terminals and bad connections.
There are many vehicles out there that were under water during Katrina or similar hurricanes.

There is a possibility that you will need a new control switch on the column.

If you use a code reader, there may be an opportunity to snoop out the codes if there are any being posted.

Good Luck
Rob