I know Carey built your car. I did not realise he was using earler turn switches or a modern non relay switch.
Ifd you do have the relay type switch
Id say The high low relay has failed. Its another case of a stuck / broke wigwag contactor. There is also a chance you may have a lose wire at the relay OR LIKE CAREY SAID A FUSE. But Im betting RELAY!
The stalk switches rarley stick. Not to say they can,t But its very unusial. In the beetles 65 were bad to break off the stalks, 66s were better but could possably stick ,,68 ups were good as gold
On the outside chance you have a floor button, they are pretty tough especaily the early Gm truck ones 51 thru 58 a good common choise for a durable floor switch.