Ok I've wired up my steering wheel controls this afternoon and tested all working perfectly.
The below instructions are half of what you need to do, as I have an aftermarket stereo I can't give the exact wiring to the back of the factory stereo.
To to this mod you will need a clockspring that supports steering wheel controls. The base model clockspring will not work there are too many connections needed.
The good news is that they all share the same plug on the steering column side that connects to the clockspring, just with varying wires going to it.
The plug in question
Connections are:
8: Horn (use its location in the plug as a reference)
4, 5 and 6: Connect to your stereo. 4 is the earth connection but that must go to the correct spot on your radio, not just any ground.
12: Illumination wire 12v
I inserted new pins into the above plug that I got from some spare Toyota plugs I had lying around. With the new wires in the plug should look as follows
Of my wires, the green is illumination (#12), red is #6, white is #5 and blue is #4.
The easiest place to connect the illumination wire is the headlight dip switch. The wire you want to splice into is shown here:
The finished plug harness is here
From the far right of the image mine continue through to where they connect to the back of my radio.
From what I've gathered from looking at wiring diagrams, the wires mostly only go to the stereo however some steering wheel button assemblies have switches such as Info and temperature controls, which obviously would run to the trip computer and climate control modules.
The phone buttons should run to the stereo but if your steering wheel doesn't have them to start with I don't know how the factory stereo (those with bluetooth) would react to having them connected sorry.
Hope this is enough to get started with.