Mine is in the trunk-right hand side underneath the Pioneer 12 disk cd changer. Bit of a chore to get out. My amp recently suffered what appears to be water damage from condensation (I think) as the drain tubes located nearby all appear fine.
I have searched to no avail for a Stereo Kit, it seems that the new system I might be looking at would have to be completely custom. As the local Toyota dealer for cost of a replacement amp and was quoted $1046 US plus taxes plus shipping.
So, thinking of a new system with a newer amp. I hope that my installer can somehow tie into the existing Subwoofer amplifier, but he is not sure. I am going to give up the 12 D changer as it is finicky about media, and seems to be starting to show its age (1995 SC400). The new head unit will hopefully have a usb plug-in as well as MP3 capable and I-pod adaptor.
This whole audio replacement endeavor has not been a easy thing, as these cars seem to be pretty unique and more complicated than many others. Any advice or comments would be welcome.