Thanks for all the responses, guys. Didn't realize it would touch off
a debate ;-)

It turned out that the guy who did the retrofurbishing of the old
Philco radio (the original AM radio for this '69 Mustang, refitted
with AM/FM tuner, CD line in and pre-amp outs) DID include a remote
wire that is hot only when the HU is on. I ran this to the REM
terminal on the amp and it worked fine.

Now the "only" issue is that the sound coming via this HU really
sounds pretty bad. I played around with the gain from the amp
(USAcoustics USp-200 65W x 2) but that didn't help much. So as an
experiment I plugged the CD unit (actually a Walkman) directly into
the amp and worked the gain up from zero. It sounded MUCH better, so
much that last evening I just ran the line out from the Walkman via
the headphone outs and controlled the volume that way. The speakers
are Infinity Reference 505cs 5-1/4" Components.

I spent a large wad having this Philco radio retrofitted because I
wanted to have the interior look as original as possible, tune in FM
stereo from it, and use it to control the amp power-up and volume of
CDs being played. Turned out that he did it as a common-ground setup
so even if I had wanted to run speakers off it the thing is grossly
underpowered. Now I've got to try getting this guy to refund my $ or
make it right somehow. I may well just get a decent modern HU and
hide it in the glovebox or the floor console (over the tranny hump).
And then just have a standalone AM radio in the dash for the retro
look.

Anyway, thanks for all the replies, I'm sure I'll have more questions.

Ken