I don't know what brand of dash cap you're gonna use, but on the ones I've had to do, the instructions deal with the installation with the original still in the car. In other words, they want you to jam books and the like between the windshield and dash top to make sure the skin is right down against the dash pad.
No matter how bad the existing dash pad is, it stays in the car. You just usually have to cut or grind off really lumpy areas so the cap fits good.
My opinion......They suck!!
But in these days of cut backs, they're the only alternative to the 750.00 repop dash.
Anyways..... I just KNOW there's a "Pro Dash Cap Installer" out there isn't there???