Have you looked at the flasher switches made by Signal Stat? The Signal Stat Flarestat (Part No. 105,106 and 107) switch was made to convert cars without four way flashers. The switch has the flasher built in and would easily accomplish what you are trying to do. Search on E bay for signal stat and you will see some listed. They also came in both 6V and 12V, but you could easily convert the switch from one to the other.
Regards,
Steve
