Panasonic CQ-DP383U
Car - 88 528E BMW
Problem - No sound during CD playback.   Sound is OK during FM play.

Today I installed a new head unit into my 88 BMW 528.   The FM radio
function works fine, but the CD player does not.   It seems to be
reading the disk, and counting down the track time, as if it was
working fine, but I hear no sound.

This was a non standard installation, as the BMW uses (I believe)
common ground to chassis for the RF- and RR- and the same for LF- and
LR-.   Anyway, per advice I found on the internet, when I installed, I
did not connect the head units speaker minus wires to anything, just
left them hanging, as the speakers are grounded to chassis.  The rest
of the installation was by the book,

The FM works fine, but the CD acts as though it is muted.   I will try
re-wiring to remove the chassis ground from the speakers and connect
them to the head unit speaker minus leads to see if that makes a
difference..  

Any ideas on this problem?   Perhaps this installation method would
cause the symptoms I see?

Thanks in advance!