I did this with my 98 with 2 amps - one sub, one 4 channel. Although you can get a scoche LOC in walmart I wanted a bit better so I got a peripheral VEN4 that converted all 4 speakers to RCA line level, then using a Y cable split the front RCA cables and fed the sub amp. Worked well. In your case you need to tap (split in two) the front L&R speaker wires and feed them to either a 2 ch LOC or an amp with line level inputs. To answer your question on the other post your factory speakers your splitting from will be uneffected.
But here is something I discovered about premium cd/cassette factory HU bass. To prevent distortion at high volumes the bass is lowered. So it all sounded good but when I turned it up past 3/4 the bass lowered some. You could easily fix it by using bass rocker on factory HU but then when you lowered the volume the bass would be louder than I liked..especially w/ FM. This inconvenience is what made me go to an aftermarket HU after a yr or 2.
Depending on what you want to do I have $60 worth of factory wiring harnesses and VEN4 sitting in my basement that I would be willing to part with.
Good choice on the Infinity Ref. They are very efficient and sound good with factory power.
