78 buick riviera, headlights don't come on

using a voltage tester you can check you have power to the switch and again a voltage tester will tell you if you have power at the outlet of the switch and also at the light terminal itself, if you have power to the switch and no power at the outlet (when on) you need a new switch, if you have power at the outlet but no power at the terminal then there is a brake in the wire or poor connection which can easily be solved by running a new wire through direct.