yea, on my second fit i used the antenna 'power out' to turn on the amp..that worked good.
low pass filter ..doh couldnt remember the name.. but same as a passive anuways only lets the frequency you want past.

good advice off the thesundancekid there.

thanks for the info.im still saving cuz i deceded it would b better to replace the rear speakers than to rush onto a sub.i was running a advanced auto box

probably just my opinion here, but i've never liked running an amp off any factory radio. Just doesn't sound near as clean as with an aftermarket ho with preamp outputs. Always sounded kinda muffled and distorted in a way. If I was you I would wait until I can afford an aftermarket ho to go with the setup. They have some now that are really good quality at a decent price. Some of them for even under $100. Well worth the investment if your looking for clean crisp sound, or slamming bass. I just picked up a kenwood ho from bestbuy(which is always over priced compared to the web) for around $150. Took 30 minutes to install. Helped the factory sound out tons thanks to a blown channel in the factory unit. Then I hooked up an alpine sub and amp. M500 amp 2 channel and the sub was rated 500 watts rms (which is the only rating that matters when it comes to watts). Best of luck and there is tons of info about setting up car audio on the interweb