It's probably the clock. I had the same problem, so I put a disconnect switch on the battery, and I turn it off when I'm not driving the car. It works fine. After 2 weeks of being off, I still had 6.7 volts on the battery. The only problem is that I have to reset the clock when I turn it back on.
Hmmmmn, no, a clock, if working properly, would take a few weeks to drain the battery. Remember there was no assurance these cars would be run every day, or even every week when they were originally sold. There is something else going on like a light staying on, a bad voltage regulator, etc. Sometimes even just a bad battery that won't hold a charge. It happens.
There are several articles on YouTube and elsewhere on how to trace a voltage draw, but a battery disconnect
is a good place to start. If you have an ammeter open the switch on the disconnect, hook up the ammeter around the disconnect so whatever power is being drawn can be measured. Start on a 2 amp or higher scale so you don't blow the circuit protection in the ammeter. For reference the courtesy light over the radio will draw about 1.5 amps on a 6V system or 0.7 amps on a 12V system. You would need a draw of similar size, I would expect, to drain a battery in less than 2 days.