Probably one of the most usefull items is a good antenna / ariel.
You should be able to buy a reasonable magnetic mount (mag mount) feirly cheaply. Obviously ensure that the coax connector will fit your radio.
You can buy dedicated magmounts for the airband frequency ie. the correct length of the whip but in practice for casual listning a cheap ham radio mag mount for the two meter band (145 MHz) quarter wave length will do ok unless you are very serious about your hobby.
Search around the ham & aviation stores some radios will actually fit into your vehicle and will recieve broadcast as well as aviation bands.
Also condider buying a little converter ( as used with MP3 players etc ) you plug this into the earphone socket of your scanner and it re-broadcasts the signal into the VHF broadcast band, at VERY low power so you can hear the RT via your vehicle's loud speakers.
Good luck, have fun
