We have had excellent results for over 5 years with the MFJ 1722 dual band magnetic mount antenna for only $14.95. It is very small and almost disappears on the car. Ours is mounted on the driver,s side rear fender of the lady ham's Mustang. I would recommend adding a piece of scuff tape or electrical tape to the bottom of the magnetic base because the magnet is very
strong, which makes it hard to remove for washing the vehicle.
If you find you need more power, I would recommend the highly rated and much loved (in our area) MFJ 1729 dual band magnetic "power gain" antenna for $39.95.
Try the least expensive antenna first. On top of the roof or rear deck, you should find that it works quite well.