Basically you have two ways to do this, I did it on a 2000 S-10 a few years back, first option is buying a convertor so to say that changes the hi-volt output to RCA type signals, Crutchfield sells them for about 30 bucks, this is what I did and it worked wonderfully, I ran an Infinity Kappa 4-channel amp, front two channels on Kappa 4x6 plates and Kappa 6.5s, rear who channels bridged to (1) 10" Kappa Sub, great little system for an S-10 while retaining the stock head unit look. This will require cutting either your front or rear speaker wires just a few inches from the harness connector that goes into the head unit. The other option is having an amp that allows you to run hi-volt inputs, thus cutting your wires close to the speakers and running them directly into the amp, could be very difficult with the Xb as none of the speakers are very close together, I would go with the first option.
the squarest looking plug goes to the rear speakers i think they used 1 black wire and 2 whites as the black is bigger than both whites and probably splits bown towards the back to be safe though you could use a mono setup and splice one white and the one black
JL's clean sweep is also an option, but if you're not familiar with car audio it may get confusing during the install.
IMO I'd go for the LOC (Line Out Convertor), and if you do get one you may want to keep in mind using the front speaker wires to use for the LOC. Reason being is just in case you want to move all the sound to the front speakers while still having the option to adjust the bass for your sub. But if you dont plan on doing that then using the rear speakers will be fine. Ive used the ones at Wal-Mart with no problems.