How old is the pickup?
How old is the battery? If the battery is over 4 years old I'd replace it anyway. If you want it for winter, you need it to crank hard.
Probably a power drain somewhere. If there is an aftermarket radio, it could potentially be hooked to constant power instead of keyed power. Some of the mid 90s Chevy's had a light under the hood come on when you opened the hood, that or the glovebox could be the problem. If not, I'd check the fuses like hometeam said.
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