Thanks Paul! Any suggestions how to easily check that the speaker wires are
getting the proper signal at the end? I can see disconnecting the speaker wires
at the radio end, and checking that neither is shorted using a battery and a
bulb. But if they are not shorted, how can I easily tell that it's amp channel
that's faulty? Connecting the speaker right at the radio output wires?

One other weird thing -- the whole problem started with the following symptom:
One morning I opened the car, and without even putting the key into ignition
(i.e., not turning on the power to the radio) I already heard the faint noise
coming from the speakers. It eventually stopped when I turned the radio on and
off couple of times, but it's not clear to me why the speakers made the noise if
the radio was off and without power!

AS

Paul Vina wrote:

Well, I was right about the speaker. Now you need to check the speaker
wiring and make sure it hasn't rubbed through and shorted to ground
anywhere. Other than that it could be the amp channel but that's rare.
Usually it'll activate the protection cicuits or kill all the outputs.

Paul Vina