I guess I should have read the manual first, but here's what my Drivers Manual says on page 24
"Light Switch (two stage)
1st stage (ignition off) - parking lights on and retracting headlights come up.
1st stage (ignition on) - parking lights and low-powered dipped headlamps.
2nd stage (ignition on) - headlights on full power (high or low beam).
The rear lights, the licence plate and instrument panel lights are on in either position; the blue indicator lights up when high beam is selected.
Instrument lighting
The instrument lighting comes on as soon as the vehicle lights are turned on. The brightness can be infinitely varied by turning the knurled knob"
So there you go, and that's just what mine do. Interestingly, at the front of the manual it says WKD924 020 88 700M 4/87 which I assume to mean it's a 1987 print of the manual, because the instructions I have copied above are on a sticker over the original text. At the bottom of the sticker it says 88-924S/P24 and amazingly this sticker goes on ...... page 24 ! So, perhaps something did change in the operation for the 1988 models and the sticker is hiding how the lights operated on previous models. I can't read through the sticker as it's quite thick paper.
Now, you may or may not be fascinated by that, but there you go.
I also note that on page 32 it show the switches down on the centre console (which is where all this started) and that does confirm three switches and one blank.
cheers ........... Phil