\nThe reason they are blowing, as soon as you install them, is because there is a short somewhere, in the flasher/horn wiring. Have you or anyone else, done anythi…ng electrical, to the truck lately, added something or tapped into the flasher/horn circuit for any reason. There is usually a common denominator, when a circuit all of a sudden, starts blowing fuses. \n.
\nI suggest you buy a circuit breaker, with the correct amperage rating and start there. Put it in the horn/flasher slot in the fuse box. You will hear the device "pop" when it blows, but unlike a fuse, it will reset in a few seconds, then it will again "pop" breaking the circuit, and reset itself again. This gives you a way to check the circuit, without using up every fuse you own. Get yourself a copy of the wiring diagram, and start at the end of the circuit, working towards the fuse box. On a 23 year old truck, good chance it's a worn wire. Eyeball the wiring in the bad circuit. The under side of the wire, the part you can't see, is where most shorts occur, as over time the insulation will wear away, allowing the wire to touch metal, and "Poof" there goes your fuse. \n.
\nTake your time, and don't rush it, finding a shor,t can be a real test of your patience. If you have to stop the work for any reason, no big deal, just leave it as is, drive it if you have to, and when you get time to continue again, pick up where you left off. ( Full Answer )