I know one "brutal" way ... Extract the windshields two dds files in the .mas. Remove all the dirty parts and make it 100% clear. I already did it by mistake on the BMW 330e BTCC. But i rapidly got back since it feels that there is no glass at all which is worst. Maybe you can just lower the intensity of dirt in some way (works like transparency in a png). Problem is it can't be done on the original version. You will have to make your own car_upgrades.mas and turn it into a workshop item (rfcmp) to run it on a server with friends.
@Ron Watmore you can make you own skin and add a cleaner windshield (inner). This should be "more" clean
