First suspect would be the amps ... those can draw considerable power on standby. Big difference between "shouldn't be" and "aren't" ...

If you've got a voltmeter, you can do a static test on the truck to see how much draw you're talking when it's parked. Disconnect the negative battery cable, set the meter to the 10 amp range, then hook the red lead of the meter to the cable, and the black to the battery terminal. Shouldn't show more than around .01 amps to keep the radio and such primed. Any more, start pulling fuses one by one to narrow things down. You'll need to wait a couple minutes so the tech stuff has a chance to go to sleep before taking a reading.

If the static test looks good, you need to get a load test on the battery. How long does it take to drain, and how old is it?

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'89 Basic Beater - '93 XLT

"As I get older, I have less and less to say. The problem is I can't stop talking."