1987 chevy s-10 p/u battery discharges

hi , i have a 1987 chevy s-10 4x4 pick-up. if i shut it off for more than 10 min it won't crank. i put a test light in between the battery and the cable to see if it lights and it did, then pulled the fuses one at a time to see witch circuit was shorting out and the test light never went out! any ideas or history of this problem in this truck. any help would be appreciated. thanks, john :banghead: