For the money that you're going to spend, a Wayfinder
wireless car thermometer is the best deal for your buck. There are additional models on the market which cost a lot more but do not offer much more than a Wayfinder. It has loads of standard options and is very easy to put in and use.
The Wayfinder wireless car thermometer sets up easily on your dashboard with the help of suction cups. It is totally self contained and uses batteries for power. There is no need for wires attaching to the dash for power and temperature readings.