The RX8 stock deck, like most factory decks, does not have pre-outs which allows you to connect a deck to an amplifier. However this doesn't mean you can't connect an aftermarket amp to your RX8. You can get the signal you need from the deck to the amp by using your speaker outputs on the deck. This is called a high-level output and some amplifiers on the market have a high-level input built-in to make installation easier. Even if your amp doesn't have a high-level input you can still connect an amp to your deck with the use of a "Hi-Lo" converter.
Disconnect the car battery to protect all electronic equipment in your car.
Remove the RX8 stock stereo to expose the wiring in the back. Consult your RX8 owner's manual for proper deck removal instructions.
Connect speaker wires to the "rear" speaker outputs on the back of the deck, if you plan on running the rear speaker of the deck. One wire to run your rear speakers and one to connect to the Hi-LO converter.
Connect the speaker wire you just installed into the deck to the Hi-Lo converter. You will install the converter in the dash so the wire just needs to be long enough that it has some slack in the dash. Connect the end of the wires to the converter, making sure you connect the positive lead on the speaker wire to the positive terminal and the negative lead to the negative terminal.
Connect RCA cables from your amplifier's inputs to the converter. With the converter in the dash make sure your RCA cables are long enough to reach from the dash to the location of your amp.
Connect the power cable to the positive lead on the car battery. It is recommended to use a power cable with an in-line fuse. Connect the other end of the power cable to the amplifier's power input.
Ground your amplifier by running a ground wire from the "ground" connection on your amplifier to a clean piece of metal on your car chassis.
Connect the blue remote wire to the amp's remote input. Run the remote wire to the back of the dash and connect it to the remote wire coming off the back of the deck. That wire will be blue. To connect the wires, strip off an inch of the insulation from the remote wire coming from the deck. Wrap the exposed part of the remote wire coming off the amp around the exposed wire from the deck. You can secure the two wires together by either soldering them together (which is recommended) or by taping them together with electrical tape.
Re-install the deck back into the dash so the wires aren't pulled so tight that they could come unplugged, making sure you have room for the converter.
Reconnect your car battery.
Follow steps 1 and 2 above.
Run speaker wire from the rear speaker outputs to the location of the amp. Connect the other end of the speaker wire to the "High-Level" input on the amp.
Follow steps 6 through 10 above to complete the installation.