Not all four speakers are functioning at a time,

It must be something with cabling. Radio should use all speakers.

 

I think you can find it in the Electrical Wiring Diagram - part of the service manual.

 

In Bluetooth hands-free mode, only the front speakers might be used, but for the other modes, have you tried adjusting the front/rear balance control, as described in the Navigation Owner’s Manual (PDF), pages 148–150, or the Owner’s Manual (PDF), pages 330–331?

Before assuming it’s a wiring problem, since all the speakers are working, just in different modes, I’d start by checking the balance setting described above.

The Electrical Wiring Diagram (more info) shows the factory-installed speaker connections, but if the car has a BLU Logic hands-free system (a Toyota accessory installed after the car was built), those connections wouldn’t be shown in the EWD or considered in the Repair Manual troubleshooting procedures.