On both ends of the main battery cables. And every other electrical contact you can get your hands on.
The fuses on these are prone to corrosion and intermittent behavior, as well.
You have a '81, so your fuse box is under the right/rear of the gas tank.
Check them. They are 8 amp fuses, and it is worthwhile to change the whole box for something using the spade-type automotive fuses. But, near as I know, those only come in 5 or 10 amp, in that range.