The horn works differently than the lights.

The horn is a switch but it doesn't have any positive current attached to it, it creates a ground that trips the horn relay on the fuse block. The horn relay sends voltage to the horn. If the horn isn't working it could be the Power to the relay, The relay, the horn switch or more commonly the steering column ground.

Under the steering column should be a black ground wire going from the stud that holds the steering column to the dash and the bracket that bolts to the steering column. Make sure it is there.

Also if the dash lights do not work check for a good dash ground. Since the E bodies had the overhead lights it is easy for the dash to be grounded but the light panel to not be grounded.

Do the gauges work? if so then the dash frame is grounding OK if not clean some paint off of the bolt and frame at one end of the dash frame under the kick panel to help the grounding.

As for the parking lights check the grounds in the trunk and behind the battery. There should be two ground wires going to the lights IIRC. Check for voltage at the fuse block on both sides of the fuse. I have seen many bad glass fuses that were not burned. Next check for voltage going in and out of the ignition & Turn signal switches under the steering column. Next check the wiring at the bulkhead.

Hope it helps.