If you are running an amplifier you really NEED to have a completely seperate wire running directly from the battery to your amplifier. If you don't, you run the risk of pulling too much juice through an older, smaller wire and possibly causing a fire.
Your installer sounds like he/she knows what they are doing, but they are just used to the "bling bling" look that the kids want under the hoods nowadays. If you want to "hide" this wire, you need to either buy or splice a new length of similar (2-8) gauge wire, and go directly from the battery, between the fenders, and then in through the firewall.
Again, I would NOT use any existing power wire for any car stereo amplifier.
