There are threeways to hook an Amp to a head unit.
One option is to use "pre-outs" which are common on aftermarket decks, and unnexistant on OEM stereos. They are RCA females coming from the deck. Some aftermarket decks even have multiple sets for multiple amps. All amps have the RCA inputs from "pre-outs" and this is the most common method by far.
The alternative option for decks without pre-outs is to use the "high-input" plugs on your amp, if it has them. Only some amps have a "high-input" option, but if yours does, you can simply run a speaker channel or two into the amplifier that way. This of course sacrificies two of your speakers though.
One more thing you can do if neither of those options are available is to purchase a "line level converter" which will convert your rear speaker channels into RCA's to use as a substitute for pre-amps. Again, you lose two speaker channels and have to pay a bit of money, but it's an option if your amp doesn't have high-inputs.
I'd recommend picking up a cheap aftermarket deck. You can get them for under $100, and that way you also get other killer features that stock decks lack such as an Auxiliary or USB input.