Dear Ian:
That is a frequent problem on the wiring of classic Mercedes w113 and similar cars. There is not a separated sub wiring harness for the speedo& tacho illumination as well for the heating controls.
It is common to have those cables chopped.
Supposing that cables are still there but hidden, let’s solve first the speedo&tacho.
TACHO
There is a pair of cables coming from the main light switch harness branch that has a gray/blue cable and another brown which is the ground. Those should be connected to the bulb holder which is behind your tacho (gray/blue is the positive 12v) and the brown should be connected to the tacho housing.
SPEEDO
On the harness branch that goes to the ignition tumbler switch there is again a pair of cables with same Colour scheme as described before. Gray/blue + brown. Do the same thing done on the tacho.
HEATING CONTROLS
On the harness branch that goes to the cigarette lighter there are 2x double prong female connectors. One of those connectors has a black/yellow/green cable+ one brown . This connector is for the cigarette lighter. The other connector in that same branch has one gray/blue cable and one brown. That terminal connector should go to the heating sliding controls illumination connector.
Look at the picture attached which has two red circles. Those are the harness branches for the main light switch and the ignition tumbler mentioned above.
WORKAROUND in the case you can’t find any gray/blue cables.
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Gray/blue cables are always related to illumination circuits on classic MB. If your car central instrument cluster (rectangle part between tacho and speedo) illuminates when you turn on the exterior driving lamps, there is a good chance that we can obtain a derivation cable to illuminate the accessories asked by you. I mean a dimmeable illumination line coming from the output of the rheostat.
You just have to open the 12 pin connector that feeds the central instrument cluster. In that connector you will have to look for a gray/blue cable and solder there the derivation cable you want to use for the bulbs holder already mentioned.
If you are not very handy soldering, there is a much easier solution but with one compromise. The lights will not be dimmeable at the speedo, tacho and heating controls. That is a small sacrifice for functionality considering the risk of doing a lousy job soldering in those connectors and affecting other circuits.
So I think this will be the best approach for an aficionado.
EASY SOLUTION
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Take the illumination line from fuse no,7 output on the main fuse box. You will notice, that from main fuse box, circuit no.7 there are three output cables. Two of them are gray/red and there is a single gray/violet.
Prepare one derivation cable that will feed the illumination on the accessories requested (tacho, speedo, heating) with an “0” shaped terminal on one side. Unscrew the cables on fuse no.7 and place on that screw the 4x cables, (2 gray/red + 1 gray/violet and 1 x new cable.).
Remember that each of the accessories which need to be illuminated also will need a good ground cable.
Enjoy your modification work.
Best regards
L.peterssen
