I used this one that I purchased at redtrailers.com:
http://www.redtrailers.com/ShowItem.asp?id=31823
There were no instructions with the unit...the leads that had to be connected to the electical system were all marked as to their function on the unit, and follow the "industry standard" color code for trailer light functions as far as I could tell. So all I needed to know was what color Yaris wire did which function going into the tail light assemblies.
The excellent wiring diagrams provided by Mustang made that a snap. I just unsnapped the rear carpeting covering the tail lights (which I needed to do anyway at some point to install my cargo net), and located the appropriate wires on the left and right side, based on the wiring diagram. The controller comes with "vampire"-style taps that allow you to connect the controller wires to the Yaris wiring without cutting anything.
Mustang's wiring diagram shows which side drives the center-mounted stop light (I can't remember if it's the left or right side now). I selected the other side to connect the brake light lead from the controller, reasoning that the side driving the center-mount stop light had enough to do already. This particular controller draws power for the trailer lights directly from the Yaris tail lights themselves (you're basically wiring the trailer lights in parallel with the Yaris lights).
Some more expensive controllers require a direct connection to the battery, and just use the Yaris tail light voltages as "soft switches" to drive the trailer tail lights directly from the battery. I don't have that many lights on my trailer that drawing the extra current through the tail light wiring would be a problem as far as I was concerend.
There was actually another gromet I found that was a little more convinient to use, since it wasn't under the spare tire. I saw it after I removed the carpeting from the tail lights. The trailer connector end is already attached to the controller, and it wouldn't fit through the grommet, so I cut the the trailer side of the wiring and just routed the wires through a hole I poked in the grommet. That's when I figured out what the butt connectors were for that came with the controller. I used the butt connectors to re-attach the wires I had routed back up through the gromet back to the controller. I tried to cut the wire so the butt connectors stayed inside the trunk, which left plenty of wire to route the trailer connector near my hitch, so I could pull it out and attach to the trailer when necessary.
So all in all I am happy with the controller. It did come with one of those neon light tester things that are supposed to light up when you ground one end and touch the prove to 12VDC. However, the one they sent my was not working. I would suggest using a good voltmeter over one of those things when testing your work anyway.
