I don't know if these cars have one, but I assume they do: a ground strap/wire between the engine/alternator to the chassis.
Check the ground resistance between the engine/alternator to the chassis. It should be close to zero. If not, connect a wire or cable (even a jumper cable will work) between the alternator's ground (housing?) and the chassis.
If your amps draw decreases, there's a problem with the ground between the alternator and the chassis. In most cars (I don't know about these--I haven't studied the diagrams), the negative post of the battery is connected to the chassis; therefore, if the connection between the alternator and the chassis is bad, the alternator will not be able to keep the battery fully charged. As the engine and the rest of the car draw on the battery for power and the alternator is unable to keep up, the battery eventually gets drained.
