If the linkage has a sloppy feel and the reverse light doesn't pop on immediately when you put in in "R". Then it may be wearing out internally (the linkage and electrical contacts) as bmw auto's have been electrically solenoid shifted since at least the early 80's. As far getting codes, sometimes the obd 2 port will do, try it. Usually though it needs a specific plug in, to the diag port in the engine bay (big round 54 pin job under the cover)