If it's an M200, it needs quite a lot of power...15V at 4A, which is 60W. Most small car chargers can't supply that, but check the specs on your Radio Shack device. Three suggestions:
1. Buy an inverter, which plugs into your car power outlet, and has a regular 110V socket on the other side. Then just use your regular Toshiba adapter to charge the laptop. The advantage here is that the Toshiba adapter accommodates a wide range of input voltages and power quality, and will clean it up for your laptop. A 100W inverter runs about $25.
2. Targus sell purpose-made car/air adaptors, some of which work on AC, too. They come with interchangeable tips for different laptops.
3. Try a Juice adapter from www.igo.com. Depending on which one you buy, it'll work in your car, on a 'plane and from AC anywhere in the world, and will also charge your cell phone and any other portable devices you may own. I just ordered their new 7500 unit (not yet shipping) which will even run off the 28V power in a private 'plane.