I'd go to your fuse block with either a voltmeter or a test light, looking for a fuse that is hot when your key is "on" and hook in there. The "cheap & dirty" method is to crimp on a male spade terminal and wedge that into the block at the end of the fuse, assuming they're the older glass tube units. If not then you'll need to trace a wire from your switched circuit and splice into it. I would not go the toggle switch route - too easy to forget to toggle off when you turn the key.