Having rewired two first gen Mustangs and now a first gen Firebird, I can safely say that you may want to carefully consider wether your current wiring is fixable or should be completely replaced.

If you're finding a lot of broken, cut, brittle wires, you'll probably save yourself a lot of headaches by simply replacing the whole section. Its not as bad as it seems. If you have a new wiring section, and mark the old one to the new one on where it all goes, its simply gutting and replacing. The key is marking with masking tape where things go, and taking lots of pics too.

If your wires are plyable, flexible, and aren't all cut up, then as previously mentioned, you go one circuit at a time.

The first place to start with is probably the grounding. Many problems are directly traced to improper grounding. Check if your fuse block is good, (getting power), check fuses, check power from fuseblock to dash area, check dash bulbs, make sure dash circuitry is properly ground, and if not...create your own grounding of that metal strip, etc. Sometimes simple wire brush cleaning on contacts can make a big difference as well.

But one section at a time. As for each component, (heater, cigarette ligher, radio, horn, etc.), you'll have to go one by one, trail and error. You maybe getting power to the unit, and the unit might be bad.