For a magnetic mount antenna, you'll need an adapter cable/fitting if the antenna doesn't match the radio. You don't necessarily need a mag mount to transmit from a car or truck. You get calls and texts while driving right? While some signal may be blocked and reception may be reduced a tiny bit, a car has enough glass for you to talk on your radio. I transmit regularly to a repeater about 12 miles away from inside a metal building. Its not perfect, but I am perfectly readable.
You losing signal with your wife is likely not a function of being in the car. I cant talk to my wife when she is on her HT and I'm on mine 1.5 -2 miles apart. Terrain and location (rural, urban, suburban) have a lot to do with range too.
Bottom line: Height is king, get that antenna up higher if you want "more Fars" (put the antenna on the roof of the car) and check for local repeaters.
Btech sells the Nagoya UT-72G for about $35, it comes with the adapter for the radio too. I've had good luck with mine so far.