Fuses can definitely develop oxidation or corrosion on the inside over time which will increase resistance drastically.
Make sure EVERYTHING at the fuse box is clean and corrosion free. Take some sandpaper and rough up the fuse tangs a little bit to clean them. Even if it isn't the problem, it's good to do once in a while.
The 82 550 C3 diagram does have some differences. The headlight uses the reserve lighting unit. The front turn signals are also used as running lights. But the ignition switch and fuse layout are basically the same as on the 80 550 A diagram.
There's a chance the reserve lighting unit is bad, but I don't think that should affect the tail/gauges/marker lights.
