This may be a long shot, but here is what I did on my 81 300TDT:
This car was in an accident and the drivers side headlight assembly was totally crunched.
After replacing the assembly, I plugged the lights back in and found that the turn signal would not blink-but it came on- or sometimes it did not come on at all. (It blinked real fast b4 I replaced it...)
So I check the connections at the turn signal bulb holder and everything *looks* OK. I try another bulb, nothing. I then try to open the little box at the end of the plug to see the actual connections (If this is confusing, I will post pics) and one of the wires had come loose. Re-soldered the connections and BINGO good as new.
It's worth a shot.
Current: '91 300TE 4MATIC 317k and climbing...
'81 300TD Wagon 168K "Tank"
'83 240D 216K 4spd manual "Da Bear" (aka best car ever)
"Never sweat the petty things...
and never pet the sweaty things."