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.


That does sound quick and easy - I've never used wiretaps, and I've been soldering a long time, so I'd naturally go with the hot stuff! I don't know how well the wiretaps would hold up to strain or time?
I trust you remembered to also connect the ground wire.

One observation that I would make is that it is not generally a good idea to do anything to the car's loom, it is always better to try and build a separate harness, especially so with a new car.
Firstly it makes things easier when it comes to selling the car - just unplug your harness. Secondly, if your car is still under warranty, modifying the original loom could invalidate that part of the warranty - so if there was ever an electrical problem on some other part of the same loom then the dealer might try to blame it on your modifications and charge you for the replacement.
The letter of the law regarding warranties is obviously different from country to country, but you could get into hot water. Those looms are not cheap. When I was looking for the stereo connector for my VW (to build my harness), I tried my local dealership. The only way that they could supply me with one was to supply me with the whole loom......$3000. Obviously I didn't take them up on the offer