Only problem you may have is if your head unit crosses over the frequency. If the head unit seperates the low signal to the sup output, you wont have that low level of sound going to the 420 since you don't have an input for the sub. See if there is a setting on the head unit that turns on and off the sub. If so, turn off the sub and you should get that low frequency distributed to the right and left RCA outputs. Then split the right and left creating the 4 outputs you need. All you have left is to wire your outputs from the 420 to your amps as normal.
2004 Supra Launch SS
Acme 913 prop, WS 420, Kicker L5 sub, JL/Kicker amps, Kicker in boats, Polk mm in roswell bar, 3000# ballast.
"Never underestimate a man that Overestimates himself."