It's most likely that there is only a capacitor (crossover) in the line that runs to the tweeter. The mid/woofer is most likely going to be playing the full range of sound. Otherwise, there would have to be a Coil inline for the mid/woofer. Most car companies only supply an x-over for the tweeter as it would blow if it tried to play the low frequencies, yet the woofer won't blow by trying to play the high frequencies.
You CAN hook up your MB Quarts to the original wiring, BUT you HAVE to make sure you use the wires that run to your mid/woofers ONLY. If the stock tweeter has a little capacitor maounted ON THE FRAME of it, then you can use the tweeter wire as long as you remove that capacitor with the tweeter!
Enjoy!
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1996 GS-R
MKIIa 30 Gig Empeg
1900 Watts RMS from 3 Alpines!