This sounds like you've got a ground issue, as several other guys pointed out. I had similar issues while wiring mine. Check the grounds for your left signal lights, your turn signal indicators on the dash, etc. It can be hard to find but I'm betting it's a ground.
I do have to admit most electrical problems on old vehicles are caused by bad ground. I don't see how a bad ground does that to lights but I guess it does. Maybe the ground is "searching" thru paths for a place to contact or some such nonsense. I forgot I used to have a motto: It's always the ground. 46G is probably right but I will stick with the switch.
