There are a lot of newer cars and trucks that cannot sit more then 1500 hours before the battery gets too discharged to start (i.e. two months). All it takes is a built-in 50-100 milliamp draw. Seems though that a 1988 would not have that much draw unless you've got an alarm-system on it.
Besides a small constant draw - any lead acid battery self-discharges even when not hooked to anything. 10% per month is an average figure. Some more and some less. The more heavy-duty the battery the faster it discharges all on it's own (due to the extra antimony).
My 1998 Dodge van can last 6 - 8 weeks before it gets dead on it's own and it's been like that since new. Owner's manual says it must be started every 6 weeks or have the battery unhooked. I just leave a battery maintainer on it.