Fogging on inside of headlight

The fogging inside might just be water condensation and you can chase it out by removing the bulb and blow hot air inside to dry it out. But on a car that old, it may have oxidized just like the outside, and I doubt you can disassemble it with anything smaller than a sledge hammer. A pair of new headlight assemblies on Ebay might be $100, which includes new bulbs.

Drying it out with a hair dryer or compressed air is virtually impossible because there is no way of getting proper air movement going. If the car is not going to be used for a few days take the light assemblies out, remove the bulbs, and lay them in a warm dry place with the opening facing upwards. Then let nature take its course. The moisture will eventually evaporate, but it could take a while. After that reassemble and you'll be good for a while. If you can see where the moisture is leaking in try to seal that, Best fix is new lights.