[Impala64ssa]

Try Glove box light (not shutting off when closed).
That's a possibility. When I first bought my 64 I left the trunk open when I parked it in the garage forgetting about the trunk light. A few days later it wouldn't start and the battery voltage was lower than Bruce Jenner's testosterone levels ;) Does the truck have an aftermarket radio/CD player in it? The clock in it should draw a very small amount of voltage from the battery when it's shut off, to maintain the time and station settings. Some of those cheap units, like those "$50 specials" from Wally World draw more voltage than they should, another thing I learned the hard way.

[babygene]

“like those “$50 specials” from Wally World draw more voltage than they should”

You meant current I’m sure...