Hello,
It is possible. However, from what you describe you should have a 10-pin connector in the harness, which is intended only for the two dash speakers.
Some double DIN radios plug into this 10-pin connector directly, some may not. Whatever model you use, make sure it is compatible with the connector. Otherwise, you may have to splice one in.
There are two side plates/gussets to fit a double DIN radio into the panel. You need to order them from Toyota. I do not have the part numbers at hand. Four screws attach each plate to the radio, and two to the panel.
You may want to replace the dash speakers with more modern units. This improves sound.
For rear speakers it is better to use an amplifier. Most amplifiers can connect to the main unit.
There is space for an amplifier in the rear inner panel, close to the rear wiper relay. From there you can route cables to the rear doors.
The best place for rear speakers is the rear doors: no need for a subwoofer, for the sound it very good in this setup. The dash/rear doors combination is a good one. You can also mount speakers in the side panels, but it is not as good in my book.
Regarding the backup camera, it is not a problem once the radio is installed.
Juan
