The installation of a radar detector is a complicated task best handled by an expert.
Some units have many elements that are supposed to be installed in different parts of the car.
Consult with friends who have had radar detectors installed by a retailer. Contact the major manufacturers of radar detectors and ask them for the names of dealers in your area. Check the yellow pages after you have compiled a list of potential dealers. The good shops will probably have large ads in which they list the brands they carry. Call the shop.