Horn and cig lighter won't work

First the horn; did the horn work when you replaced its fuse? If not, try swapping the horn relay (located in the inderhood fuse box) with an identical one from the fuse box. If the horn now works, the relay is bad.

Now the lighter: Unplug the lighter and then install the 20a fuse.
If the fuse doesn't blow, then the light assembly is the problem (this is common on a lot of GM cars). Get another lighter assembly.

Let me know what you find.