Hi @jurgenkoppen ,
Don’t know the answer but I was wondering if it were possible to unplug each [https://www.manualslib.com/manual/804776/Bmw-Z3-E36-7.html?page=48#manual|component] one at a time to see where the problem is emanating from.
maybe start with the [https://www.manualslib.com/manual/804776/Bmw-Z3-E36-7.html?page=316#manual|Board computer -low A35y] as you suspect, which is located in the centre console.
just a thought.