Connect an ammeter between the battery positive terminal and the positive lead - DO NOT try and start the car!
With the ignition off you will see some current draw indicated on the meter.
Pull fuses one at a time until the current draw drops or stops. That will narrow down the circuit on which there is current draw.
Do you have an after market stereo or alarm? People often wire them incorrectly so they drain the battery.
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My '80 progress thread - hugger orange!
Now with LQ9 power - forged pistons, LS2 rods, 236/242, .621/.612, flowed and milled 317 heads. MS3X
4L80e, 12 bolt, 4.11, Moroso brute strength posi, steel shafts.
Subframe connectors, g-braces, solid body bushes, carbon hood and front bumper, custom door cards, Autometer gauges.