I tapped that little red wire into main battery power. That way I can use the compressor at ALL times even if Jeep isn't running. Hell, sometimes in the garage when I check pressure, one or two tires are slightly low, so I just air them up without starting it.
With that being said, We just installed a seat heater on ours and wanted it hooked to IGNITION only. I spent about 30 minutes trying to find a good ignition source that was easy to access with no luck and just used the cigarette lighter. It's really tight in there, so if you use that, you should solder it and shrink wrap it.