I'd be looking for a bad ground connection. Bad grounds will cause all kinds of wierd, crazy and unexpected occurances. The really wierd ones happen when current from one application searches for a path to the ground and since electricity takes the path of least resistanace, it will sometimes try to ground itself through a different application. A good example I've had happen a number of times when pulling a trailer is when the ground on the trailer fails to make good connection to the truck. The result is when applying the brakes, the brake light is very dim and the tail light comes on dimly, even though the tail lights aren't even turned on.
On the '70s pickups, there are at least two places where the dash wiring harness grounds to the metal dash. In addition, on the fuse block the positive side of the fuses are connected to two buss bars (a strip of metal that provides a common connection), each of which is connected to a single hot wire. If your fuse blick has two rows of fuses, you might check to see if there is any commonality between the affected applications. For example, if the horn, brake lights, ABS and instruments are all located on one row of fuses while those on the other side all function correctly, that could be a clue that the problem is before the fuse block rather than after. Most often, but not always, a bare wire results in a blown fuse or smoking/melted wiring because they tend to ground out on whatever hard object they rubbed against, which is usually metal. Non-conductive objects tend to be plastic and aren't hard enough to wear through the wire insulation.
To summarize, you might first take a look at the fuse block to see if the problems have any commonality. You may also one at a time unplug the fuse to the afflicted function. For example, unplug the horn fuse and see if any of the other problems disappear. If not, replace that fuse and pull the next one, even those to unaffected functions. Pulling a fuse may cure the problems, or a function that operates intermittently may cease to function altogether. For example, if pulling the horn fuse makes the brake lights stop working altogether, even when they are supposed to be working, that is a clue that a bad ground connection somewhere was causing the horn circuit to be used as the ground. If the fuse stuff fails, then grab a flashlight and peer under the dash, wiggleing wires and see if anything starts working the way it's supposed to. When it comes to electrical problems, it's easier to find the ones that quit working altogether. The intermittent ones are the head scratchers.
