Don't know if this is relevant but I have a BMW 740 Sport (E38); this model develops all sorts of faults related to the Light Control Module. This is a box of electrickery that supervises (!) all the gadgetry related to the dashboard instruments, warning lights and steering column controls including the cruise control. It is susceptible to water ingress and when that happens it's a throwaway job.
Don't know whether the B4 cab has such a module but if so, might be a good place to start.