Never having been around a 64.5, I'm not sure if they had a hydraulic stop lite switch at the MC.  Butt  (TS&T), if it does that's the first place I would check.  Back in 64-65 when I was a "highly trained auto electrician" at Sears and Sawbuck, I bet I replaced a dozen of those on various cars over a couple of years.  Some times they will actually keep the brake lites on but not always.  I've seen  a number of them fail and pass just enough current to run the battery down over a few days but not enough to heat up the brake lite filaments.  If that isn't it then try the courtesy lights if you have them, then horn circuit.  Also, as I recall, the voltage regulator on the old generator systems can fail and keep power going to the field circuit. 
Chasing sneaky shorts to ground is a real pain so I hope this helps.

BB1
 

Last edited by Bullet Bob (9/06/2023 6:20 AM)

A fully charged healthy battery should read about 12.6 volts.  They call, then 12V, but they are really 12.6 (6 cells of 2.1 volts each).  IME anything showing 12+ volts sitting static is healthy, provided it passes a load test.  Older batteries will start to drop some voltage as they age.  You might be able to throw them on a charger and get them back to 12.6V, but after sitting for a day or so they will drop back to somewhere around !2. 

To test for a parasitic draw you should check amperage (not voltage) between the negative cable and battery post with the cable removed.  Anything over 50mA is too much and indicates a draw somewhere.  Once you see what your draw is you can start trying to pinpoint it.  Pulling fuses is a good way to find the circuit causing the draw.
 

The potential for the radio to draw down the battery depends on the radio.  My car can sit for months (my WRX is the same) and the battery is barely effected.  I would think that either the draw is so incredibly low that its inconsequential, or that manufacturers finally got smart and started saving those presets to some kind of memory which doesn't require power. 

TKOPerformance wrote:

A fully charged healthy battery should read about 12.6 volts.  They call, then 12V, but they are really 12.6 (6 cells of 2.1 volts each).  IME anything showing 12+ volts sitting static is healthy, provided it passes a load test.  Older batteries will start to drop some voltage as they age.  You might be able to throw them on a charger and get them back to 12.6V, but after sitting for a day or so they will drop back to somewhere around !2. 

To test for a parasitic draw you should check amperage (not voltage) between the negative cable and battery post with the cable removed.  Anything over 50mA is too much and indicates a draw somewhere.  Once you see what your draw is you can start trying to pinpoint it.  Pulling fuses is a good way to find the circuit causing the draw.
 

This^^^^ is where I like to use a test light. (with the door closed or tape the courtesy light in ) and everything off...... start pulling fuses until the light goes off. "there's ya sign" !
6sally6
 

Didn't have the car door open while performing the test, did you?

I should have mentioned to check that the glove box light isn't on all the time, and any trunk, or underhood, light that may be installed. 
Regarding charging the battery for two days: 
When you checked the voltage was the charger still attached?
If the charger wasn't attached, how long after it was disconnected was the battery voltage checked?

Check the charger voltage without it being connected to the battery to verify the charger is working properly.
If this is OK, check it with the battery connected, if the voltage is at the 10.6 volt range, seems like you've got a shorted cell in the battery, and it should be replaced.