My 78 had the same symptoms. It should be an easy fix. With a fully charged battery, installed, check for voltage (I like to use a heavy duty test light with a long sharp probe, and a good alligator clip ground) at the panel where the 12v
attaches to the bus bar. Attach the alligator clip to the BATT NEG at the bus bar
Then work your way down checking both ends of each fuse, and the bulb in the handle should light up. Even if there is a fuse element in the glass tube, does not mean it is good. Pull each fuse out and clean any gook or dirt from the fuse holder as you go. I use a product that ATT uses called NOOXIDE. It's a brown grease that prevents oxidation between dissimilar metals. There is a prouduct at Lowes called Alumalox, I believe that I have used as well . It only takes a dab on a qtip, and a very thin coat at that. Here are two things that can aggravate you if you don't check them. 1. Just because the fuse is intact (passes visual) doesn't mean its good. I have had several old fuses and new ones that were OPEN, even though I could see the fuse metal.
2. Every once and a while pull one end of the fuse out and push it back in, top and bottom all the way down the line.
3. The most overlooked part to service. Check all of your ground connections at the panel and at the battery. Make sure that they are clean and free of corrosion and verdigris. A dab of your favorite anti oxidant or petroleum jelly to finish it off and you should be back in business
It takes about an hour and a half to do it.
Let me know how it goes for you, and if you have any other troubles.