There are OEM TOYOTA Stereos available on ebay.
The Steering wheel controls go to the remote CD changer plug on the OEM stereo.
This port is used ONLY for the steering wheel controls on our Sienna as they did not offer a remote CD changer on the Sienna.
The CD changer for the Sienna is inside the radio unit (when present).
You load the CD's one at a time in the CD slot.
Most (mine included) have the single CD player......and the CD Changer version has a "load" button next to the CD slot.
This picture shows the back of the OEM stereo in my 2003 Sienna.
The plug at the top of the picture (left side when looking at the front of the stereo) is the plug with the wires from the steering wheel controls.