This is one problem with one car. I know, if it's your car it is the biggest problem in the world...but. This new Mustang has it head and shoulders over my old SN95 (1996 GT). It handles better, rides better, has more room, super performance and the quality overall is much improved.
Maybe I've been lucky, but mine hasn't had any problems yet, and it's a 'vert. No rattles, no cowl shake. I had to put a lot of extra bracing in my '96 to get it to be as tight as the '05 is right out of the box. It's a great car. Best I'ver ever owned.
A decent dealer should be able to take care of a squeaking dash. If they don't, turn them in to the regional service people or to Dearborn.