You might require another AC compressor and all the plumbing required. You will have to retrofit vents and then network everything together. I am not quite familiar with the plumbing work required but I will say that it isn't going to be easy. You will be the first I've heard of doing something like this. Seems interesting.
I am in no way saying that you shouldn't do it, I just do not know of anyone who has ventured into this modification. I would actually be really interested in seeing this done. You can always crack your rear window for them
The reason I mentioned the AC compressor would be because if you just add ventilation networks to the rear and link it to your current A/C unit, you will experience a drop in air pressure through your front vents.
My idea would be to run the rear on its own system, completely. This way you do not have to interfere with the already, tried and proven AC system in the 4Runner, currently. Maybe mount an AC compressor in the rear, near your spare tire mount, run the plumbing up and mount some sort of control system in the rear to control temp, fan speed, vent control, etc.
The price could be daunting and its practicality might be limited, but will that stop you?
