"If there is a problem in the left channel, it
will be heard in both the front and rear speaker. "

Not necessarily anymore. Many car radios now have a separate amp per speaker. The advantage is if you like bass you can crank up the rears and they can distort slightly and the front will play clean. seems like more power but it really isn't.

Delco has been doing that for a long time. In fact some of the higher end models also have anti-clipping.