R129 199 sidelight and horn issue

[DrNick]

Dear all

Whilst prepping my R129 for MOT a couple of issues came up.

1) Horn not working (mostly). Occasionally it makes a noise. I'm suspecting a bad earth. Can any one let me know where the horn is and how to access on a 199 SL500. Or if anybody has any direct experience about what it is if not a ground issue. I haven't investigated the voltage at the relay yet.

2) Rear side light drivers side. Bulb is ok (swapped with other side which was known working). Reading on wire (no bulb fitted) varies from 3V to 11V direct to battery negative, seems random. Same when measured from the unplugged light connector block. Earth on the wire block connects to ground ok. Looks like something on the positive side is not happy. I don't believe this model has a SAM, but the light fault indicator on the dash lights up so the car knows a bulb is out. I haven't noticed any other electrical systems not working but TBF haven't checked everything. All other lights/indicators work ok. Battery is ok. Car runs well.

Anybody got access to a wiring diagram? Can't seem to find one on the web that's readable.
Just gathering opinions and ideas before I delve in and start taking apart all kinds of things I probably don't need to!

 

[DrNick]

Update:
Horn: TLDR - I replaced the horn. All good now.
Light: TLDR - After a week of being left it mysteriously started working and seems solid

More detail on horn:
Horn is located under the bumper in front of the drivers side front wheel. Access is by removing the road wheel and removing the plastic cover that would be in front of the wheel. Easy to do. It is a two tone horn with 2 snail shell elements bolted onto a mounting bracket. There is nothing special about the horn and I replaced it with a Amazon cheapo one as the Halfords one I bought had had the mounting kit stolen and there were no others in stock. The only issue was that the original screw connectors on the horn had badly corroded and so couldn't be undone, so I cut the wires. On stripping back to fit a crimp, all the copper wire elements had oxidised and gone black. I rectified this with the old tomato sauce trick. Coat the copper with tomato sauce and wrap it in kitchen roll to slow it drying out and leave to soak. Other reducing agents are available but tomato sauce doesn't dry out very quickly. It has a lot of vinegar in it which is the active part! After doing this for a few days (between weekends) the copper came up clean. New crimps and new horn fitted and works well. I used this one https://www.amazon.co.uk/dp/B09CCZR7WP?ref=ppx_yo2ov_dt_b_fed_asin_title&th=1 working so far.

Light: Well, we will see. It just stared working. After it started working I looked at the fuses anyway (all good and clean), and cycled the switch a few times in case there was something there, but no - kept working. So currently its working fine and I can't make it fail. No doubt it will again soon, but the MOT is tomorrow so hopefully it will hang together until then.

 

[DrNick]

Just to close this out, it passed the MOT with some advisories
Ball joint dust cover perished but intact (replace)
Suspension components surface rust
Brake pads worn at rear
Front brake discs worn (slight lip)
Front wheel bearings slight play