Easiet (not always the best) would be to put in a line level converter which 'piggy-backs' off the signal from your stock stereo and turns it into a line-level signal good for use with most amplifiers.
Some four channel amps can take in the four channels straight from a radio (no line level), amp the 4 speakers and then send a stereo signal out to another amp, if I remember right.
I guess if your looking for the easiest way to put in a sub, so then I'd get a line out converter and splice it in to feed an amplifier. Which amp/sub are you installing by the way?
I live in my own little world, but it's ok.....
they know me here.
-2000 XLT Super Cab 4D 4.0 OHV 5-speed
Hidden Sub 'Mod'
-1959 VW Karmann Ghia Convertible 1776cc Dual 40 IDFs
1996 Suzuki Katana GSXF 600cc Gas Mizer