Does your alarm system work? The round plastic connector is probably the connection to the door handle. Most of the time this is unplugged to compensate for a malfunction in the alarm system.
If you have never had the heads off, it is past time for a top-end rebuild. It will cost about $2500-$3000, but it sure is better than replacing a whole engine. The bottom end almost never goes bad.
When was the last time you had maintenance done on the suspension? The self-level needs to be babied, i.e., frequent fluid changes, etc. Is it leaking?
1985 500SEL (260,000 miles) SOLD!
1974 280 SOLD!
1994 Volvo 850 Turbo Wagon (Yuppie Ice Sled)
"I know I have a Benz and they know it too." If you start bragging about it or shoving in people's face, you then become a Wally; especially to the girls.