I've got a Bel 985 and I have it mounted on the windshield and powered through the cigarette lighter. From my experience, the best way to set up a radar detector for mazimum effectiveness is to have it so the the radar/laser sensor is in full view of the rear windshield. If you put it too low on the windshield it can cut the effectiveness of the rear dectection especially.
If you plan on hardwiring it, I guess you'd want to be mounting it up high. From what I hear that works good too, as long as it still has view through the windows.