For A/C to work the way you expect it to, the cold must blow in your face or close to it. Roof mount may not do what you want. Smaller units take less space but tend to freeze up easier. I am using the mini version from Vintage Air, in dash unit. If I don't keep on top of the temp knob, it will freeze up fairly quick. Around town, it blows super cold air, out on the road it will freeze up so I need to warm it up, then cool it back down. It works well this way, but I have been told the bigger units keep steady temps better.
