Did you make sure the lock cylinder in the trunk lid is not switched to the valet mode is the slot vertical or horizontal in the lock cylinder? The release mechanism for the fold down rear seats unfortunately is located inside the truck. There is a release knob to pull just on the inside edge of the rear hinges for the trunk lid. Check the valet mode let me know what you find.
If the key is in the horizontal position this is the valet mode. Do you have another key to try? If you only have a valet key it will not allow you to move the lock cylinder to the vertical position. This is why the inside release cable is not functioning it overrides that cable when the lock cylinder is turned to the valet mode. You need to get a non-valet key to turn the lock cylinder to the vertical position or to just simply unlock with the lock cylinder. If you have no other keys available it will be very difficult to go through the back seats if you in fact have fold-down rear seats. The other option would be to provide proof of ownership at a local Toyota dealership and they can cut you a new master key not a valet key based on the vehicle identification number and the keycode registered in the system.
You're very welcome. If you need anything in the future just answer me, skyvisions, in the question and I will get back to you.