A way to check battery drain without first disconnecting the battery.

Since it is difficult to effect a decent electrical/mechanical connection by trying to insert a milliammeter between the battery and it’s associated bolt-on connection, to say nothing of all the wild current surges that go on when you try to reconnect the power to the circuit, I think I have devised a reasonable and electrically clean (no big sparks) alternative method to accomplish this test.
First, remove the plastic cover over the fuel manifold, it just lifts off, to expose the alternator and its’ output terminal.
Then remove the little plastic cover over the alternator output terminal (if it is still there), it also will lift/pry off.
Oh, be sure that you have the car doors fully closed and the ignition and lights are off for this procedure. Do not forget the little courtesy lights (two 194 bulbs) on the bottom of the rear view mirror. The light under the hood will only light if the headlights (or parking or fog lights) are on but the light in the trunk will light anytime the deck (trunk lid) is open. Could be a bad mercury switch? You should be able to watch the light go out as you slowly close the trunk lid.
Next connect a good 12-Volt source, I used a fully charged small motorcycle battery, negative side to any convenient ground. Or you could use a small battery charger I think.
Now, connect the positive side of the 12-Volt source to the positive side of an ammeter. I used a Radio Shack # 22-812 digital meter, (same as a Fluke meter.) It has a 10-Amp internal fuse and on the 10-amp scale, it will read down to the nearest 1/10th of a milliamp. (.0001 mA)
Then connect the negative side of the ammeter to the alternator output terminal using an alligator clip jumper wire. No need to remove the wire if you know that the alternator is good. A good alternator does not draw any appreciable amount of current. (On the order of Microamps, m)
Now, you have two voltage sources feeding the car’s electrical system.
You will finally need to disconnect the Positive side of the car’s battery and then you are left with just one 12-Volt external source powering the car’s electrical system through the ammeter and through the alternator output wire which is connected to the positive cable of the battery which is connected to the power distribution system to the rest of the car. So now, you can read the “Quiescent” current draw, which should be 20 - 30 mA (.020A to .030A) or a little higher if you have RKE (Remote Keyless Entry.)
You can also use this method for trouble shooting if you make your wires to the ammeter long enough to bring the meter inside the car with you where you can begin pulling fuses and/or circuit breakers. I would first disqualify the lighting circuit as the culprit and then disable it since you are probably going to have to have the passenger door open to access the interior fuse panels. There is also another fuse panel under the hood next to the air cleaner.
Pull the fuses one by one until the high current draw goes away to isolate the circuit that is causing the drain on the battery, then trouble shoot that circuit.
I know there are a few things that do not go through the fuse panel. Like the Anti-Theft circuitry, and perhaps any aftermarket devices that may have been added, those will have to be checked individually.
So, that is my method for reading the idle current draw on any Reatta. Or perhaps, any other kind of GM car? I hope I did not leave anything out or if anybody wants to add anything, please feel free to do so.

Wow. Thank you.

Impressive. The Steakmobile has a current draw that will kill the battery inside of two weeks. I could arc-weld with it (just kidding). When I hook up the battery I hear a clicking coming from the relay center by the air cleaner. Is that normal? Steak

Steak, I believe that the click that you hear is normal but, I don't know what relay that is. If you follow the procedure above, there should be no click since you did not open the battery circuit to insert the ammeter in the first place. I hope this helps.