that diagram took me 2 hours, so i hope you appreciate it!
go to jaycar, DONT SHOW THEM THAT, they will laugh at you, tell them what you wanna do and what kind of switch you need, then wire the ground from the head unit to the single input, then ground one of the outputs, and wire the other to the ground cable on the handbrake...
finding a suitable place for the switch is always fun... i got 5 wittle ones in my drink holder