The standard tweeter wire connects to the underside of the tweeter. You'll need to pop the tweeter out of it's housing in order to reach the wiring clip and disconnect the wire. You may be able to reach the clip without fully removing the tweeter from the dash (the hard bit being squeezing the tweeter out from between it's mounting clip and the actual surface of the dash)
The "custom fit" or "perfect fit" pioneers that people refer to are a set of component speakers, not a headunit. You can get them from MT Audio on here. My point was that whilst you might buy decent speakers, they're still being powered by the same pathetically small/cheap amplifier which is built into the standard head unit. Buying an aftermarket headunit would enable you to make much better use of the standard speakers.
Of course once you have an aftermarket headunit you will then probably get the urge for better components because the aftermarket headunit can probably provide more power than the standard speakers can take without distortion... My point being you can't solve sound quality or volume just by replacing the headunit or the front speakers, but you can improve it a lot for a relatively small amount of money just by grabbing some aftermarket stuff.