As a bonus, a simple magnetic mount antenna on the roof will give you significantly -- sometimes dramatically -- improved range and clarity over using a stock antenna inside the vehicle (aka Faraday cage). However, if you're going to regularly use the radio with an external antenna, I recommend you get a BNC adapter to reduce strain on the SMA connector. You'll also need a BNC to SO239 adapter to attach the radio to the antenna. SMA connectors weren't designed to have antennas screwed and unscrewed too many times and they can break with frequent use. BNC connectors were made specifically for a great many antenna swaps.
I have BNC adapters on all the radios I use frequently so I can quickly change the antenna according to the mission. For example, most of the time, I can use my favorite repeater with an 8" antenna, but if I'm going to be a little further away, I change to a 15" whip. There's a 1.25 meter repeater in Oklahoma City, but I can't reach it from my usual places, although I do go there sometimes. I don't want to have a 17" antenna on my HT all the time, so I can quickly swap the shorter antenna for a tri-band when I'm near the city.