Well if you got any old “RCA” cables what i would do is cut the end off of one and wire it to the back speakers and then you plug them in to the amp and then hook up the power supply for your amp and then connect your subs and that should work?
NOTE: THE ABOVE SOLUTION WILL NOT WORK AND WILL DAMAGE YOUR AMP!!!
A typical Head Unit will push between 10 and 50 watts to each speaker, while the RCA inputs on your Amplifier will only take about 3.5 watts. In Stereo World, this is called “Input Voltage.” You need a device that will lower the amount of power coming out of your Head Unit, so that your amplifier can receive an acceptable amount of incoming signal. In short, DOING THE ABOVE WILL ABSOLUTELY FRY YOUR AMP.
What you are looking for, friend, is commonly called a “Knockdown,” or a “Step-Down-Converter.”
This is a device that wires directly to your Head Unit IN PLACE OF a speaker (READ: you have to sacrifice a speaker chanell to do this!), and lowers the speaker signal from the head unit to “Input Voltage” level, so as not to fry your amp (hence the term “Knockdown”). This unit will be about 3″x3″x6″ and will have RCA outputs on the back of it to hook up to your amp. Find one here:
PS: Any Mom and Pop Stereo Shop should be familiar with this item, and should have some in stock, they are very commonly used in your situation, and are not new to the industry.