Are you talking about the "Under Dash Pocket Mount"? The one where it is just a plastic bracket that you can unclip the unit from? Are you going to be switching the P3 from TV to TV or do you want to remove it for some other reason?
The metal bracket is what most use. You can use the screws to mount it to the dash or you can even use industrial Velcro. If you use Velcro make sure to clean the dash plastic with some rubbing alcohol first. You have a lot of leeway on the angle from front to back on the P3, just make sure it is lined up in the line of travel.
Keep in mind that the small metal bracket and the plastic holder will stick out farther from the dash vs. the large metal bracket. My knees would probably hit it if I tried to use that mount.
Mine is flush-mounted in a pocket on the dashboard. I just used Velcro as the fit was pretty tight in the first place.
'15 XLR 27HFS
'15 Nissan NVP 3500 SL
Campers - Me, DW, 3 - DS (7, 5, 3), 2 large dogs