Erik is correct that there are three screws and a backing plate on the bottom of the dash to hold the mirror in place. When I put my dash covers on about a year ago, I removed the dash from the car. The real secret is trim to fit very slowly. If you take off to much you will have a gap, not enough and the fit will be bad. I enjoyed the experience once, but if I ever had to do it again, I would set aside a week that the car could be down.

I really think that to fit the cap correctly the dash should come out, but I am sure there are those who were able to install without removal of the dash. I found that there are actually very few wires behind the dash and as long as they are labeled correctly the process in not too bad.

Good luck, which ever method you try. If you have question feel free to contact me via e-mail.

Oliver
1966 188S
Columbus GA