how to wire tweeters?

Have you tried moving the tweeters in your component set first?  If you're dead set on adding tweeters to your current setup, try to stick with the same brand, and only use one set.  Also, unless you are using an amp to power your speakers, I would not recommend doing so- headunits are not meant to run anything below 4 ohm speakers. 

If you are using an amp, ensure that your amp is 2-ohm stereo stable.  Any add-on tweeter system should come with it's own crossover.  Wire this in parallel with you components before the crossover.  Don't try to use the component crossover to do this- you will through off the impedence, and the crossover points will be mismatched.  Play around with the positioning of the tweeters until you find something that will work best for you.