How to fix a Prius 2001 dead speaker

Seems likely that the speaker has failed. Can you access the speaker mounting hardware? If yes, then remove the hardware holding the speaker to the shelf, and then you should be able to access the wires leading to the speaker.

Once you have removed the speaker you can look at it. If it has not obviously failed based upon visual inspection, use an ohmmeter across the speaker terminals to see if there's continuity across the speaker coil.

If the speaker is OK, then set your multimeter to AC volts and measure voltage across the wiring connectors when the radio is on. If no voltage then you'll have to pull the radio and see if AC voltage is being output, or maybe the wiring is bad.

 

Lacking an ohmmeter etc., the more rudimentary way to confirm that a speaker's voice coil is intact is to connect it to a 1.5 volt battery. If the speaker makes no sound its 'fuse' has blown.

 

In that case it might be necessary to remove the parcel shelf trim while you are perched on the rear seat, in order to replace the speaker. Maybe you'll need to access the repair manual info at techinfo.toyota.com to see how to do this, without breaking something...