Well it's not the only way to make it work, just look at the guys with 00-01 s2k's. Since your S came with seperates, ideally you would replace them with seperates. Having your Infinity coax's going and the factory tweeter will mess with your sound quite a bit. Either leave it the way it is w/o the crappy factory tweeter functioning, or pull out your coax's and drop in a pair of seperates.

 

You can't just plug in the replacement tweeter in place of the OEM. You have to use the crossover that came with the component set. Just plugging in the replacement tweeter to existing wiriing guarantees that it will blow. There is no resistor or crossover on that circuit, just a direct feed from the headunit.