Switch doesn't handle enough amps

i was told i need a switch because the "switch +" is not hooked up to the iggnition. so it would constantly be on all the time

so it would potentially drain the battery?

There is no need for a switch.  The reason HU have the ignition wire is so radio goes on and off with your ignition, so you dont have to do it manually.  But, if you are going right to the battery, dont worry about it, just use the button on the unit itself.  10g wire is completely un-needed.  I would run 16 guage, with a fuse rated about 5 amps higher than your HU(because of power loss in distance, if it is real close to the battery, make it closer to the size of the HU's fuse.  Run the wire to the 12v+ AND the Ignition wire of the deck.  Then connect the Ground wire of the deck to something metal, or the other terminal of the battery if there is nothing else metal.  And you're done. 

BTW, you are over-thinking this whole thing, wich is making me overthink it.  Wich is LAME!