Just a silly question, but is it doable?

I have been thinking of setting up an alarm system with a knight rider type scanner light in front of vehicle. What I want to do is have some kind of motion sensor inline so when there is motion the scanner will do a full scan until motion has gone away. The scanner will then place itself in normal mode and do a scan every once in a while. I am almost positive that this can be done, but don't know where to start. What alarm would be able to handle something like this, what motion sensor(s) to get, and extra circuits that may be needed. Has anybody tried a project like this? Am I just dreaming of it being possible? Can it be done?

A rader sensor placed on the front of the vehicle that tripped a timer ckt (I think there is one on this site) would do this. The trouble is the current radar sensor are made for inside the vehicle and hate heat (radiator) and weather (not waterproof). If you have a plastic front end and put the unit far from the radiator and wraped in so water couldn't enter it might work.