Update:

I let the car sit in the driveway this morning with the key in the accessory position for 10 minutes and the radio off. Started perfectly

Same with ignition in the on position.

Next, I played the radio for 10 minutes and killed the battery.

WTF!? How much current does a transistor radio DRAW?

So, it seems to be that circuit. Any ideas?

Interesting....a short, low amp recharge takes care of it? Do the battery cables feel warm when this happens? If not, I'm beginning to wonder if instead of a current draw problem, you have a starter problem. My '96 Buick Regal had a starter go bad where it would occasionally sound like a low battery charge. The little bit of discharge of the battery during the 10 minute period may be just enough to limit what the starter needs.

Power antenna?

Replaced it with the manual one several years ago. Went through two and got tired of it. Radio reception is better, too.

Let me restate my Twilight Zone BS from a different angle.
Voltage (14.6) and capacity are not always the same. Had you charged the battery up when it was load tested... or did it come straight out of the car without any messing with it... just as it would be when you run to the grocery store?
- If there are any battery cable issues in the charging path, it LIMITS the current going IN to the battery.. can limit a 200 amp alternator to a 2 amp charging device (ie trickle charger).

But if you want to chase a mystery power drain, you need to verify that it is actually pulling current to the max while in ACC. That will take either a very high current capable meter (you would torch a little volt ohm meter) ... or ... you can put a 12v load in series with one of the battery cables while in ACC.

You can drain a fully charged battery in 10 min. with a WHOLE bunch of current draw.
OR
You can drain a poorly charged battery in 10 min. by listening to the stereo.

Which radio do you have?

AM/FM/CD. Standar one, not the optional.

I'll do the test light check tonight. Isn't it interesting that the car runs fine otherwise? :?

The most either radio can draw is 20 amps ( 2 different 10 amp fuses). What happens if you turn the headlights on for 10 minutes with the radio off?

Are the vacated wires from the power antenna secured and not grounding out against anything in the trunk? Your issue might not be with the radio at all.

I was just doing some calculating on my StarTrek Tazor Calculator. To pull down a fully charged 850 Cold Cranking amps battery in 10 minutes, it takes something north of 40 amps.
Is that wire insulation I smell?