Make absolutely sure your meter can handle the load. I have no idea what that would be, but with batteries supplying some 120A (I suppose most drain is at cranking) they can be high. If you're using a multimeter, start in the highest setting. Most have overcurrent protection, but some isn't all that great and I once bought a second hand Simpson that had an aluminium rod (looked like a potentiomenter shaft to me) instead of the fuse , and it's relatively easy to smoke a cheap meter.
Most batter drain in these cars happens because the ignition switch fails with age.
Remove your console cover to expose the switch capsule, and pull off the thick gray wire at the 12 o'clock position (caution: it's always live). Put your ammeter between this wire and its contact on the capsule. Make sure the key is turned to "off" and that the courtesy light isn't on (close your doors).
Measure the current draw. If you have accessories (alarm, say) that are wired into the ignition system, subtract their current draw to see if there's a substantial remainder. If so, most likely the ign. capsule is bad.