So after a trip to yellowstone, I re-discovered that driving my wifes 08 Yaris long distance is a pain in the ass.. literally, and legs. SoI did some digging around on the intertubes.
Found out that the 09 Yaris is INCREDIBLY easy to add cruise control. Just plug in a stalk into the steering column, and your done! The 08's don't have the wiring already in place, as cruise control was not offered as an option that year. But the ECM programming is there!
After some further research on the yarisworld forums, I found that pin 40 of the A21 connector on the ECM is the key. This pin when directly grounded, switches the cruise control on/off. When grounded with a ~630 ohm resistor "Sets" the speed. There are 2 other resistance values that accellerates, and decellerates by 1mph increments, but I decided to not use these.
Ran to radioshack, bought a couple of resistors, and 2 pushbutton switches. The hardest part of this entire thing was figuring out how to get a single wire through the firewall into the dash from the engine compartment (ECM is located there). Finally got the wire through the main harness grommet using a long piece of coat hanger wire. I decided to give it a test run before drilling holes in the dash.
I first pulled connector A21 from the ECM, and inserted a wire into the unused pin 40 hole. Care was taken to not short to other pins, and to still get a solid contact. Once clamped back down, it held solid. I then quickly twisted a 630 ohm resistor to the other end in, and handed it and a ground wire to my wife in the driver seat. I then remembered there are 2 pins in the fuse/relay box that need to be jumpered temporarily to replace the clutch safety switch that doesn't exist.
We took it down the street, first touching the ground wire directly to the "hot" from the ECU to turn on the cruise. Then got to about 40mph and tapped the ground to the resistor to "set" the speed. SUCCESS!! The car cruises along at its set speed like it should!
Now all that needed was cleaning up the wiring, and installing some push button switches.
Here is the result:
Still needed to be completed is a real clutch safety switch instead of a jumper, and some sort of indicator light to tell you the cruise is on.