I didn't measure the diameter of the two holes but guessing, one is a round hole about 3/16" diameter. The other one is behind this one and it is a rectangle hole about 1/2" wide and 3/4"-1" in length. I had to use a dremel to make the rectangle opening larger so I could fit my Wilson 1000 3/4" mount into it.
In order to lower the rear of the headliner enough to install the new antenna, I did partially remove the rear upper plastic pillars, basically enough to lower the rear sides of the headliner. Then I removed the 3 upper tabs for the headliner and was able to pull it down enough to get my hands in there. When it comes to mounting the actual antenna, it will require 2 sets of hands. Hope this helps.
Thanks for the quick reply ! I was afraid there might be 2 holes. And the rectangular one really scares me. I don't feel like tackleing the roof mount project, and taking a chance of screwing things up. I think that I'll just stay with the magnetic mount for now.