Some people mount them to the floor or wall of their trunk. Other people mount them to their subwoofer box. I have no idea why they mount it to their sub. I don't think it would be too safe for the pc with all the vibrating. If I had a pc to mount in my car, I'd go for the mount on the side of the trunk or behind the dash. I am doing a laptop install in my car, though. Here is an example of a total in dash install.
Hope this helps!