well.. if it has a built in amp.. I assume that the wires from the sub, are connected to the amplifier.
So what you should do, is go out and buy a cheap amp wire kit, and find out where to hook the amp up to the battery, grounding RCA-and- remote wire
so.... look in the back of your tube and figure out what wires you can plug in.
I think you probably need the following wires...
1.) Power (wire from the Positive terminal of the battery, to the amplifiers power input)
2.) Grounding (from amplifier to a spot on the metal frame or chassis of the car. Scrape off some of the paint, and try drilling a small hole, and attach the wire to the bare metal)
3.) RCA (the infamous red/white connection, like the back of your tv or playstation. have the RCA wires into the input of your amplifier, and the whatever in the back of your cd player.. mine was non-fading)
4.) Remote Wire (usually small blue wire, where you should attach it or connect it to the remote antenna wire. If there is not power antenna wire, try the power wirein the back of your cd player. It is usually blue)