low voltage to radio?

[Andy Schor]

My aftermarket Blaupunkt radio sometimes won't play when turned on.  The
lights light up and it cycles through all of the functions on the screen
(without being prompted) while the faceplate is attached and the unit is
turned off.  When I press the power button, it lights up but there is no
output (sound).  The CD player shows the track and time displays, still no
sound.  The red power lead is connected to the relay panel
up under the dash (behind knee bolster), the yellow lead is connected to
the yellow in the harness.
I'm not an electrical engineer, but is it possible that there's enough
juice to turn the unit on, but not enough to power it?  
Something else?

Andy Schor
'91 200q20v Avant