The dash switch is a non-latching momentary switch, which switches a wire from the BCU to earth.
This signals the BCU to switch the heated windscreen relay's coil to earth, which it does for the timed period.
During the timed period, the BCU supplies a live to pin 5 of the dash switch, in order to illuminate the switch's tell-tale LED.
Check that the dash switch's earth, on pin 4, is OK - if the switch's night-time illumination works with sidelights switched on - then, the earth is OK.
You will also need to check that the BCU has the heated windscreen option enabled - you'll need an appropriate tool to do this - Nanocom, Hawkeye, etc...
.