Hi, everyone; 1986 420 SEL: the power wire in the fusebox which supplies the ACC blower got hot, was touching the cable from the temp sensor tot eh display; melted through the insulation and gave it 12-14 volts. Smoke came out of the instrument panel!
I repaired the blower wiring by installing a separate, 25A fuse; however, now I am repairing the temp display. I got a used one; I need to repair the harness; it is burned in the fusebox, about the last 6 inches before the connector. Will it work OK if I splice in some new wire and solder the joints, then cover with heat-shrink tubing?
By the way: my car had never had the factory-recommended upgrade to the blower wiring: the problem is, because of the high current the blower uses on "High", and the amount of time the blower spends on "High" in warmer climates, it can overheat the terminal ont he bottom-side of the fuse block, causing the terminal to loosen and be pushed down by the spring tension holding the fuse. Better check to see if your car has had this; there's a lot of wiring in the bottom of the fuse box.

I would think that your idea of soldering in some new wire should work.
The thermistor operates on a fairly narrow resistance range, but a few inches of copper wire shouldn't make much of a difference.
I wonder if my 1984 300SD ever had the blower motor wiring upgrade?
How recent was this?
I wonder if it is a safety recall that MB might do for free?

__________________
Bob Roe
Lehigh Valley PA USA
1973 Olds 88, 1972 MB 280SE, 1978 Datsun 280Z, 1971 Ford T-Bird, 1972 Olds 88, 1983 Nissan Sentra, 1985 Sentra, 1973 230.6, 1990 Acura Integra, 1991 Volvo 940GLE wagon, 1983 300SD, 1984 300SD, 1995 Subaru Legacy L wagon, 2002 Mountaineer, 1991 300TE wagon, 2008 Murano, 2007 R320CDI 4Matic 52K, some Hyundai, 2008 BMW 535xi wagon, all gone... currently
2007 Honda Odyssey Touring, 2014 E350 4matic

Just open the fuse box and see if the fuse for the high blower is still in service; the upgrade was to replace the existing fuse with a separate fuse in a holder, wired to the blower circuit. You can remove the two Phillips screws and lift the fuse panel; the blower wire is attached to the bottom fuse terminal with a Phillips screw. Take it off and attach a fuse holder; M-B recommended an increase to a 30A fuse, but I kept the 25A. I did not want to take too much chance with a 20-year-old car's wiring, and the problem was caused by the terminal overheating and melting the fuse block, not by blowing the fuse. The 25A has worked fine for me. You can buy a fuse holder at a parts store; I used a typical modern auto fuse. Simple. You can buy the "official" fuse holder and fuse at a dealer if you prefer.