I have no idea...
But, there are some people who might be able to help you out.
You could talk to those people at
www.crutchfield.com. If you're looking for a USPEC cd changer, that'll be compatible with your factory receiver...they should be able to tell you if the wires for the cd changer are preinstalled or not.
maybe, toyota dealer as well.
My question is, does your factory receive have CD changer controls? If not, then I doubt it would have the wires pre-installed for a cd changer.
Usually, most cars don't have the wires for a cd changer pre-installed if there isn't already a cd changer there.
also, take a look at ths:
http://carstereohelp.com/strtoyota.htm