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Using a fuse block, there are some spare circuits for future gadgets.  Three of these are switched.  I ran ignition from the dash, and the main power from the amp (shares the amp's 40 amp fuse).  A Jeg's terminal block made things more tidy, and easy to rewire in the future.

The fused circuits are run off of a relay.  Ignition on to trigger the relay.  The switched source is the main power.