My 1986 190E 2.3-16 central locking system suddenly stopped working, and I ended up tracing the power line to a location under the driver's side (US car) footwell. Over the years there was apparently enough of snow on my feet and other sources of water/dirt to fill a channel where the wire went under the flooring so that a crack in the insulation on the wire led to corrosion of the copper. The copper was literally pure copper oxide at one point and jerking around on the wire would intermittently allow the solid copper ends to contact. I cut out the bad section and replaced it, then wrapped the whole section in electrical tape. It has worked for 6 years since.
The point of this discussion is you have an intermittent connection and until you trace the wire you will be depending on some kind of luck to discover the problem. In all likelihood the area of the problem is within a few feet of the wiring harnesses you were pulling on, so I would trace the wire from the tail lamp back to that area. Hope this helps, Jim
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
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)