what is the usual cause for a parasitic battery drain on a 1993 grand marquis
There isn't a "usual cause", any electronic component can cause one, any short to ground can be a parasitic drain. Look up a few how-to's on finding parasitic drains. Just FYI, constantly opening a door will keep one or more computer modules in an active state and drawing more current, so if doing a test with a meter you may want to take that into account, or pull a fuse for the door so when you keep it open to access an interior fuse box it's not causing anything to remain active while doing your test.
Don't forget to test the alternator, sometimes the diodes go bad. Look for things like trunk light not going off. Radio.
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