Wirelessly posted (Mozilla/5.0 (iPod; U; CPU iPhone OS 3_1_2 like Mac OS X; en-us) AppleWebKit/528.18 (KHTML, like Gecko) Version/4.0 Mobile/7D11 Safari/528.16)
My suggestion would be on the roof as long as it isn't too tall and wouldn't hit things above. The kind of antenna would depend on what frequencies you are monitoring. Placing the antenna on the roof would be the easiest because you could run the wire through the door or window. If you placed the antenna on the trunk, you would have to run the wires under the body of your vehicle.