I've got an even easier way. For my "double-din" install (OEM Head Unit/Empeg - 1 amplifier), I plugged the (geez, what do you call it.?), the empeg harness (with the bare wires) into the empeg ISO connector, and then used some wire taps to clip the 12volt constant and switched wires to the wires just behind the wire harness that plugs into my factory headunit.

Took about 5 minutes. And no soldering. Also, you can get 10 wire taps for about a dollar.