As with before, you will have to be the one to choose what you want to add. Which subwoofer is right? Do I need another amplifier? These questions are valid ones and I will do my best to help answer them. As for your subwoofer, I would recommend that it be of the same brand as the rest of your system, but that is only my preference, you can choose any you like. Now you have the subwoofer you must decide do you require another amplifier to run it. If the amplifier you have is large enough to run both your subwoofers then a second amp is not required and you can wire in the new sub as recommended by the install instructions that came with the amplifier.
But you want power right? Sure you do, because you purchased your amplifier specifically to power the one subwoofer. So you can either purchase another amp specific to the new sub or a large enough amp that can power both subs and replace the smaller amp with the new larger one. Your call. If you are going to run two separate amplifiers then you should go and purchase another amplifier installation kit. I strongly suggest you do not just splice into your existing power wire. However it will be fine to splice into your remote wire connection.
As for the RCA connection, you can purchase a RCA splitter and will not have to run another set of wires. Of course not all players are the same, so have a look to see if your deck possibly supplies two separate RCA outlets (some do). In which case, you may want to run the separate set because it may affect your sound quality if your player resources are not properly utilized.
Your ground wire should be ran separate but it may be ran to the same location as for the first amplifier. In case I did not mention it before, your ground cable should be of the same gage as your power supply cable. It is just recommended practice and I would have to agree to do so.
Ok so now you have ran all your wires the same as before and spliced into what you are able to, time to give it some sound. Turn your volume settings low and connect your power and turn system on. Once again, as mentioned in a previous article, adjust your sound using the dials and switches on the amplifier. Like what you hear? I truly hope so and stay tuned for more installation guides by yours truly.