No wires to begin with!

If you are going to run amps it is better to use battery cable both Pos and Neg to a fuse block / splitter.  If you use more than one amp get a multiple fuse block.  This allows you to fuse the amp(s) accordingly.  Also if you are using multiple amps you can put in different fuses for differrent amps at the fuse block. 

The deck can be powered from your regular fuse block.  Remember to run a 28 gauge wire from the head end to the amps.  Normally a blue wire (from the deck) that will turn off / on the amps when your deck is turned on. 

You can also run voltage from there if needed for crossovers, pre amps etc.

Whoa!  28 gauge wire!  Haven't used that stuff in a while!  Typically what we use is 18-20 gauge primary wire cuz it's not as fragile as smaller stuff, but big enough to get the job done.  As for the remote turn on connections, be aware that most head units cannot support the turn on of more than 2 external components. so if you get maybe 2 amps, an eq, and a crossover or something then you'll need to run some relays to safely turn on all the components.

When you said "ground, constant, and switch" did you mean "switch" like the +12 volt switched connection?  or did you mean an actual switch?

Accessory (switched) wire can be found in the ignition switch harness.  It's supposed to be an orange wire.  +12 volt constant is red, but be aware that if you tap in to the constant connection there at the igniition harness, you might blow the ignition fuse.  Better to run a fused wire directly to the battery.  Ground connection can be any bare metal behind the dash that tests as a good ground using a DMM.

Even if there's no radio currently in there, there should be some wires still there.  They might be tucked back in there somewhere, or I suppose they might not be there, but it'd be easier to use them than to run all new wires.  The original wiring is constant= orange, switched=yellow, ground=black.