What gauge does the Mfg recommend for the power supply? Probably 8 ga?
Rule of thumb is you run a positive line directly to the battery and fuse it within 8" of the terminal. Fusing is a must, no exception. For ground, you can find the closest ground to the amp as you can find. Again use the proper gauge fromt he Mfg. You want the ground as short as possible. Use a solid ground like a seatbelt bolt or seat bolt that is directly bolted to the chassis. Also you want no paint in between that connection. If there is paint, remove it before clamping down on it.
Amp wiring is very specific to the amp Mfg. I am running a 4 ga line from the battery into the cabin and fuse/split to 2 x 8 ga for my 2 amps. My Fosgates require 8 ga. Again each time you step down in gauge, you fuse that line.
I am also running a home run (electrical wiring term) for an additional +12v from the battery. Basically a 10 ga line fromt he battery (fused) into and under the center console for use in a relay switch. The HU remote out "flips" the relay and the +12v power goes as a turn on to my 2 amps and Sirius connector. That way the +12v powers the Sirius connector and 2 turn ons for the amps. I want the HU harness to only supply constant +12v and swirtched +12v to the HU only - nothing else. The wire gauge in the harness is tiny and I don't use that for any additional.
For your purposes, the speaker signal will be the amp remote, correct? Then all you need is a positive and a ground wired. But fuse that +12v!!!
Hope that helps...