I too have noticed this- I have just ordered a dash mat from
California Car Cover and hope it will help. I suggest a dark grey if you have
the grey interior, The darker the mat the less light it can reflect.
Well I've never cleaned the dash with anything but Windex, so the plastic still has its factory surface. I did some testing with various materials today in the midday sunshine and the only material that doesn’t produce significant reflective glare is a flat black.
The windshield is angled perfectly to project the dash’s reflective image back onto the inside of the windshield… it’s a mirror of the dash in the midday sun. Even if the dash is a flat gray it still reflects.
So the next project is to find a way to “paint” the dash flat black. I’m thinking I might try a chunk of black felt…