I'm just throwing this out there, but I have seen a shift away from typical 12v on/off wiring in newer cars. I know for a fact that just trying to get an aftermarket keyless entry into my pickup required taking it to a local sound/security shop. The door locks were not even 12v and required relays and resistors to trick the pickup's computer.
I'm betting that since everything now runs on a CAN-BUS system that can control multiple systems on one wire they don't want people just tapping into a wire because they think it runs a taillamp.
Side story: While working at a BMW service center, I had a gal come in with three "unrelated" issues that all seamed to pop up at the same time. Her parking sensors would go off every time she put the car in reverse, the windows wouldn't do the 1/4" drop to open the doors (framless windows on a coupe that fit into a chanel on the roof), and radio reception would almost dissapear. This would come and go at random. After looking at the car three different times (all of which the car worked fine as soon as we could get it in to look at it, with no stored trouble codes), we finally got to look at it while it was acting up. The tech was able to see that there was a low voltage at one wire of the parking sensors. An online conversation with one of BMW's help center folks in Germany got us a wiring diagram for each circuit that was acting up. None showed they were related, however the tech being pretty observent, noticed that one of the CAN-BUS comm wires was shared among all three systems. He located the wire and started tracing pins. It led him to a streach of wire behind the dash, pulled the dash and found the one wire common to all three that talked to the main ECU was pinched between the dash structure and a reinforcing beam. It had been randomly shorting to ground, and something with this system thought that voltage drop on this line was a normal operating condition, so all three "brains" acted as if it was normal.
So in short, with new wiring systems, just knowing what wire goes where might cause more headache if you try to cut/add anything.