Its the bare copper wire that should be in the middle of the valve cover with the coil connections. Each side has one in the middle of the 4 ignition coils. Under the BMW plastic valve cover covers.
86 325es (wrecked by careless driver)
87 325 300k (project for life) aka 1BADETA/Christine
89 525i 190k Winter/Drift Poutana
00 540is 6spd 223k (daily driver) new motor at 214k due to chain guide failure