Ian, I would suspect as the powertrain is obviously BMW then the ECU's and BCU's (body control units) will be CANBUS and quite complex.
The switch is quite remote from the actual lights circuit and is just used to trigger a signal to the BCU, which in turn then actually does the 'switching'
I would try and seal up as many gaps in your garage as you can access and bung a dehumidifier on for a week or so, hood retracted, bonnet open and top of the fuse box off too
