I believe the ground connections of concern for instrumentation are behind the instrument panel. You have to remove it and check the grounds on the pod itself and their connection to the frame inside.

As for turn signals, the problem may be the 4-way flasher switch. I am not entirely sure about this for W123s, but on W201s the way the system works is the blinker stalk "signal" to the blinker relay goes through the 4-way flasher switch. If the switch is in the "4-way flashers "on"" position, the signal from the blinker stalk is disabled so the relay doesn't try to 4-way flash and make a turn signal at the same time. As they age, when you turn them off sometimes they get stuck before they get back to the "enable" position, and the 4-way flashers are off but the blinkers are still disabled. You can try exercising the switch back and forth and try to push it a bit harder into the off position. I did this and got it to work, but then when it failed again, when I pushed it hard I broke something. I took it apart and found I had broken the little lever that pushes the contacts back into the blinker enable position. So, I bought another from FastLane and the problem was solved.

As for windows, the switches on the console are usually the culprit. Given the 4-way flasher switch is on the console too, it seems someone may have spilled something on the console (no cup holders back then) that has gotten into the switches and gummed up the works. I believe if you do a search here you will find explicit instructions on how to get them out and disassemble, clean and reassemble them. Or you can buy new ones - I would be tempted to get them out and see if there is a sign of some contamination on them.

Good luck, Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)