[Eric Coffey]

David makes a good suggestion. If the problem changes to the L side by switching inputs, then it is obviously a receiver (internal amp) problem. Also, you mention that the "other 3 speakers continue to work just fine". If so, they might be on the same (parallel) circuit, causing the internal amp to overheat/shutdown due to low impedance. Anything below 4 ohms per channel is (generally) too much for a standard internally amplified car receiver to handle. Either way, an external amplifier is always better than the misrepresented internal power of most receivers. They can get away with listing your head unit as "40 watts per channel" due to the fact that it is 40w at 10-30% THD!