That model has bullet connectors in its plug junctions which are soldered from the factory, The plugs are very easily disassembled and a new wire can be soldered to the old bullet connector.
Ive remade quite a lot of these type of wiring (mostly for W124 due to its flaking issue) - the W126 did not have a wiring degrade issue like the W124
It seems that your "mercedes" auto electrician is useless.
If your original wiring is completely burnt out and the connector plug clamshells are also melted away or completely missing then it makes sense to look for a new harness, but even then you can buy the individual plastic plugs or salvage it from some other harness (all old benzes used the same wiring plugs) and repair the original wiring.