When my electric problems occurred, they were also caused by the trunk harness, its the harness going along side the left hinge that holds the trunk on the car.
Open you trunk and look at the left hinge, you will see couple of wires in black tubing. check every individual wire, for me, when I opened it, i noticed as nice gash right through it. Stupid design really, it gets pinched every time you close your trunk...
1992 325i / Briliant rot - SOLD @ 301,000km
1998 318is / Arktic silber - SOLD @ 207,000km
2007 335i / Jet Black - CURRENT @ 28,000 km