The type of wire doesn't matter much, solid is better only because it keeps its shape. I just finished soldering 2 pins to a new coax wire, tomorrow I'll run it to the bumper and make an antenna. From the reading I've done it seems that the stock antenna is a terrible design so basically anything should be better.

I miss the Compustar system in my old car, even from a block away without a clear line of sight it worked 100% of the time, and that was with the antenna inside the dash and not at the top of the windshield like recommended. If I can get even half that range I'll be more than happy.