That socket is, I believe, a cd changer connection.
Pinouts here (at the bottom of the page). Ignore the 6 pin connections (and probably the wire colours).
You'll get quite a few google hits with 'toyota 12 pin audio' or similar
I understand that if a toyota compatible device (cd changer, MD player) is connected here it can be controlled through the standard front panel buttons & show track No etc. The problem is that it recognises devices & controls them through a data bus connected to TX+ & TX- / pins 9 & 10. This data bus is a Toyota only standard called AVC-LAN, based on NEC IEBus. I'm having trouble finding information on these.
This unit appears to add Aux-In using this method. There are also protocol convertors which convert Toyota data to Alpine, Kenwood etc.
I'm interested in this as its an OEM type solution, doesn't involve hacking the wiring about and you don't need an external switch mounted somewhere. There is also good potential for using the radio display.
As far as the connector goes, I believe these Toyota parts nos may apply :
90980-10513
"Housing, Connector" = Housings Male and Female
82998-12340
"Terminal, Repair" = Sockets for Female connector with 6" pigtails.
82998-12330
"Terminal, Repair" = Pins for Male connector with 6" pigtails.
Stolen radio repair parts - not sure if you'll want sockets or pins?
Let us know how you get on.