eracode:
Eventually we realised that HUDs don’t work with polarising sunglasses.
Presumably this HUD is an LCD device. LCDs work by transmitting/blocking polarised light. If the LCD's light is polarised at right angles to your sunglasses' polarisation, you won't see anything. If you put a polarising filter (e.g. from a camera, or a sunglasses lens, or a computer screen anti-glare filter) between the HUD & the sunglasses, with its polarisation at 45 degrees to both polarisations, you should be able to see the HUD again.

