How much room have you got for access??
If the head is shot completely, the only option may be to drill the head off, leaving the short length of screw shank proud after you remove the flange which should come out with a good pair of vice grips.
May sound a stupid question but have you used a Philips screwdriver instead of a Pozidrive.
When you come to refit, suggest you get a Stainless cap screw (Allen screw).
I think it's either M5 or M6 around 20-25mm long and remember to put a washer under the head and some anti sieze on the threads first.
If you are removing it with the dash in place, I know how pushed for space it is you need to be a Gynacologist!!