STOP right now before you cause a fire in your car.
The amount of power a wire can carry depends on its gauge, the length of the run and the fusing. If you do not know all three of these items and why they matter, do not hook anything up to your accessory wire, particularly the amplifier.
Most car amplifiers require a dedicated power line that is properly fused and of sufficiently heavy gauge to carry the load of the amp. The FAQs section has some good reading on determining the proper gauge wires for the load you require.
Accessory wires in cars are usually not capable of carrying a heavy load and are most commonly used in aftermarket applications to signal an amplifier to turn on or a car PC to start up and so forth. The signaling function draws very little power. The actual electrical load for these devices is properly carried on a separate line.
You need something like a car amplifier installation kit, but you also need to figure out how much power you will require so you can get the proper sized kit.
Edit: there is some talk related to this topic on a parallel thread
here