Vacuum lines for leaks. First place I would check would be top of engine for vacuum leaks. Then follow your lines from there. I have a 68 and I know there are some differences but not sure what. Do they work with the engine running? Start tracing you vacuum lines looking for leaks. Engine to light switch, to reservoir to vacuum motors on lights themselves.
Ya the switch is a big failure point as well as the cracked and leaky lines. You should be able to push the manual release on the switch and if it's operating you should hear either hissing or a change in engine idle or something. Best thing to do is check power to it with a meter like someone suggested. Then you just start tracing leaks through the vacuum lines from the engine back through each connector.
Just a heads up, your vacuum pots may not hold air any longer. Those old seals go dry and crack and thats all she wrote. If you pull your line off the.. I think it's top of the can and it has vacuum, you should hear a big hissing and feel suction on it when you actuate the lights.
Talk to Don or Mr Vacuum if you need new cans. There is a good write up here by.. someone.. on swapping over to a relay system but it's a bit of work and certainly not original.