I may be a day late and a dollar short here, but in order to run the tweeters directly from the head unit, you are going to have to wire the factory amp to be driven by RCA outputs on the head unit. This will require some wiring modifications to the factory amp wiring, and while you are there you may as well seperate the front and rear inputs and use both sets of RCA outputs to tailor the sound even more to your taste. Its not difficult, just a bit time consuming.
The way I did it was just get a set of RCA cables that was made for connecting a 4 channel amp to a 4 channel deck. I clipped one end of the connectors off and wired that directly to the factory amp after tracing the wiring, using the same diagram I believe Max posted up earlier. Then just take the front speaker output from the deck and run new wires to your tweeters in the doors. Disconnect the factory wiring to those speakers so you don't have 2 sources trying to power at the same time. It may be a real pain in the butt trying to get the staging/fading adjusted properly that way though. Remember the factory amp has speed adjusting volume, so the door speakers and the factory subs will get louder the faster you go, while the tweeters will not.

Another option is to wire the Kappa speaker and tweeter in series, thereby increasing its ohms value. It will require modifying the factory wiring, but shouldn't be hard to accomplishmat all. Its fairly easy once you understand the concept. You will take the factory positive input and connect it to the positive side of the Kappa woofer. You will take the factory negative input and connect it to the negative side of the new tweeter. You will take a new, seperate wire and connect it from the negative side of the woofer to the positive side of the tweeter. Bam, wire in series, bringing ohms up to probably 8. System is happy, you are happy, minimal mods speed volume adjustment works on all speakers. Easily reversible too.