There was a tech article posted on the HAMB regarding this a few months ago. I was interested in doing the same so I saved the link. Unfortunatly the pics are no longer in the original post but someone re-posted the schematic on post #34.
Its slightly more involved but still pretty simple and I think it does what your looking for. He used a column mounted signal lever in his example but I believe a simple toggle switch would work with his wiring.
http://www.jalopyjournal.com/forum/s...=566803&page=2
Regards,
Bob NJ