The Sony CDX-GT300 is an auto receiver with a CD player, AM/FM tuner and powerful built-in amplifier. If you want to connect the CDX-GT300 to a subwoofer, however, you'll need to tap into the receiver's preamp outputs to send signal to the sub, where the subwoofer's amplifier will power the speaker. One simple connection and a cable is all that's required. The connection is the same with all car stereo receivers, so you can take what you've learnt and apply it to other models.
Plug one end of the RCA patch cable into the subwoofer preamp outputs of the CDX-GT300. The preamp outputs are on an attached cable connected to the rear of the receiver. They'll be marked with a tag that says "SUB OUT."
Route the cable under the carpet of the interior of your vehicle to the subwoofer location. Make sure you get a cable that's long enough--about 16 to 17 feet--to reach the sub.
Plug the other end of the RCA patch cable into the inputs of the subwoofer. These will be on the control panel of the subwoofer and will be marked as "PRE-OUT IN" or "LINE IN."