I don't think those error codes are much to worry about, except the first 255 error- that may, or may not, represent something very strange: Mostly likely an 'undefined or unexpected' problem, as you may well know. If it has EWS- so uses a transponder key/RF ring around the ignition barrel, the last two errors may be saying the car does not recognise the code. Have you tried programming the key to one of the 10 key memories? In DIS, you can see each one used, was your key shown in the list of keys trusted by the EWS/ECU? I suspect your root cause is actually preventing power to the second module/antitheft box under the rear seat, amongst other things around the GM. But the antitheft module was an optional feature, so yours may not have it. Either way, power from the rear distribution unit/GM/antitheft probably goes to the OBC and EWS module with the blue connector beside the steering column, beside the knee bolster. This disables the starter motor and the ECM from running fuel and spark, IIRC. But not windows etc.
When was yours built? IIRC, EWS II is 6/95 (depends on production/market). Are you sure yours has EWS? Is it EWS I or EWS II? If so there is a module under the dash. Supplying power to one of its pins allows the engine to start.
What diagnosis tool did you use? BMW EasyDIS is very good for e34 via the ADS interface.
Have you verified the body electronics (50A) in line fuse is not hairline cracked/rusted/blown? This is not by the battery, it's under the rear footwell carpet, drivers side on a LHD V8.
One solution is to strip the seats and carpet out get a new body wiring harness from a good e34 (same build/same options) and fit that, or at least to replace the left side and everything around the rear power distribution box. Repairing the cracked/oxidised wires is fraught with pain- there are so many small connections. How is the state of the power distribution connectors, especially the GM ones, under it? If oxidised around the pins, resistance can build up down the wire (water can seep down each wire around the copper strands a long way, causing oxidisation and resistance which may be a root cause for a failing GM or DWA, let alone the connectors rusting). If you have EWS II should be another module sandwiched in between the seat support and the rear power distribution box, you can only see it when you unbolt the plastic 10mm nuts holding the distribution box and pull it back out a little to find see where the antitheft/alarm module is hiding. It is not mentioned in the official EWS technical info like this http://www.unofficialbmw.com/images/BMW_EWS.pdf
I had a later 1995 520i here for a while, it had locking and starting problems too> The wipers, radio and windows, may have worked or not (I can't remember) but to make it lock and drive I disconnected the antitheft module. I never tried removing the GM.
PS. Without your supplementary wiring, do your wipers work on full speed? I have a euro 1995 540i (7/95) here with the same problems you describe, except its wipers work fine on full speed setting. I haven't tried to run it without the GM, yet. To make it run pull one of the pins high on the EWS unit (it may have been pin #8 in the blue connector, I can't remember for sure), this enables the engine to start and run. No windows, no locks, no slow or intermittent wipers. Right now, I have the interior out, and am replacing the body loom. It is taking far too long though, as I'm building a house, and that takes priority!
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