On the Bose system I'm amazed you can even tell if the rears are on or off, the rears are unbelievably fisher price.
Is it dependent on anything i.e. bumps in the road or heat?
Bumps would indicate loose connection but it's more likely a dry solder joint on the amp as the Bose amps are very prone to this. When they cut out, if you jack the volume up do they come back on?