[JoulesWinfield]

Are you saying that you want a power wire for the main power on the amp that is off when the key is off?

Thats not the right way to do it.
On any amp worth wiring there is a remote wire that you should connect to switched power, either off the stereo (usually blue) or find your own.

Your main power for the amp better come directly off the battery with its own fuse within 24 inches of the battery.

You cant wire anything additional through an existing circuit without first doing your home work (check wire sizes, potential damage to other devices etc.).

The only other way to do it would be to have a relay capable of handeling the load of the amp then wiring the coil from a switched leg and power from the battery.