Here's how I did it for mine: I may be wrong, but I
THINK I'm right
You have two separate systems that require a switched power source. If you've never installed an amp, they require two powers: one big-arsed main one from the battery and switched from the deck.
The big ones' reason is obvious, the switched one is so that way the amp turns off when the deck is turned off. No drain on the battery this way.
The reason that there are two of them on the Porsche is that the antenna for the radio is built into the glass. You can't have an antenna like that without externally powering it, so it also has a switched power source required.
How I connected it with my stereo: Out of the aftermarket deck there should be (at least one) a blue wire signaled for "amp/remote turn on." I connected both antenna/amp leads to that blue wire.
Also note, that you will most likely need a box to connect your stereo to the speaker wires. You can't just plug the speaker wires from the deck directly into the amp. I used this one cause I bought my deck from Crutchfield:
http://www.crutchfield.com/S-wA0oGlqmc3u/App/Product/Item/Main.aspx?I=127OEM2
Hope this helps!