Dennis fairly well spelled it out. However, there are a few things you want to keep in mind.
The MFJ device might allow you to operate from the accessory socket, but you still run the risk of overheating the vehicle's wiring. Although the power ports are often fused at 20 amps, the wire size is inadequate for long term use even if the draw averages 10 amps, and this doesn't address the voltage drop therein.
The connectors themselves are suspect at any current level, as the relay on spring pressure. The last thing you want to do is start an electrical fire. Others might suggest using a fuse tap, but this too is an unsafe practice.
Small spirally wound antennas aren't very efficient, and using a mag mount adds insult to injury. And there is an insurance issue with using them as well.
The old fashion notion of tossing a rig into the car, and taking a trip isn't safe or prudent. A fact everyone should consider before the fact.