Not necessarily. Voice coils can lock up and then they are silent.
As for BOTH rear speakers being stopped working, here is what I recommend: Check the wiring with a Digital Multimeter (DMM). If you are unfamiliar with them, here is a crash course.
Take the radio out. Unplug the harness. Take the red lead and attach it to the positive wire for the right rear speaker (purple wire). Take the black lead and attach it to the negative wire (purple w/ black stripe). Set the DMM for ohms, the upside down horseshoe looking thing, or Omega. Should see approximately 4ohm. Do this on the green wires as well. Should also be 4ohm.
If you get 0ohm, select the continuity setting on the DMM. Looks like this °))). Put to the purple wire, and then touch the other lead to rim of the cig lighter socket, or another good ground. If it beeps, you are grouded out somewhere. Check the wiring all the way back and fix. If not, then something is wrong with the wiring. If that is the case, run a set of speaker wire to the speaker and test to see if it works. If it doesn't, get new speakers.