Bongo Fury

I was working under the steering column (in the pic that is the break peddle going off to the bottom right) and saw this switch with Blue/Green labels and a min<->max dial:
The wire behind it seems to go around to the left and disappears into the centre area under the automatic gear selector (I guess the engine bay).

I briefly turned it on/off - I didn't notice anything happening

I assume you have aftermarket white dials.
I have seen these used to adjust the back light colour and brightness.

Turn your dashboard lights on then use the control it will change the colour and brightness of the backlighting. Its an after market addon https://visual-performance.co.uk/mazda-bongo-200-c.asp