still have a stock tape deck in a el fairmont do i need to get a new one to hook an amp to it i wanna keep the original speakers but add a sub...

i have forgotten alot of installation steps about putting subs and amps and alike... put i am sure it is nigh impossible without the RCAs

when wiring up an aftermarket head unit, you will wire up all the speakers anyway. There is no special trick to keeping the stock speakers. They are a simple 2 wire connection, just like the aftermarket speakers (one is positive, the other is negative)
Wiring up an amp is pretty straight forward. You need:
*Suitable power wire from the battery to the amp
*Suitable earth cable to the body
*Amp trigger wire, runs from the H/U to the amp
*RCA leads, to tansfer the signal from the H/U to the amp
*Suitable speaker wire from the amp to the speakers.

The suitable thickness wiring depends on the ability of your chosen amp/sub. decide on your amp/sub package before working out what size wiring you need. Also when choosing wiring, decide if the package is the last one that you will want for this car, or if you will probably upgrade later down the track.

Yes, you will be able to add a sub and amp keeping the stock head unit.

As mentioned, the amp needs power, ground, a trigger and signal.

Power and ground are obvious. Get an AMP with "high level input", that was you can just tap into the rear speaker wires, often this will act as the trigger too.

If you wish to use an amp with only low level (RCA) input, get a High to low level convertor, this takes the signal from the speaker wires, outputs it to RCA connection and away you go.

Usually it would be a poor solution due to the loss in sound quality, however only running a sub off the amp, i doubt it will matter for obvious reasons.

Its actually pretty simple!

This is what i did, works well. Standard EF Fairmont H/U, running JL Audio splits, and a kenwood amp running a sub.

thanks guys thats awesome

whats the input ratings for a high input sub is it 6.0 volts?