Buy a Super Cat from Japanese manuf. Yupiteru. . you can clip it to the sun visor. .it's that light and small (the size of a Motorola Startac). I've been using these detectors for the past six years.
http://nex-computers.tradenote.net/p...Cat-Radar.html
It can be solar-powered, and has a motion sensor for power saving.
There's also another one integrated in a mirror.