what kind of crossovers do you plan on running them? xo's are made according to specific drivers, not for any speaker with a + and -. But to make things easier, most car audio xo's are 4 ohms. If you just want a tweeter in your system, then read below.
If you're unsure of what you're doing, go for a component or coaxial set. If you think you can handle it, then I say try going for the cheap but really great sounding aura's that "desertheat" is selling on the forums. DOn't look down on them, they are awesome sounding tweets. It would be "compatible" with most car audio passive xo's in the sense that it's 4 ohms, and has the ability to play down to 3kHz or so.