On my '74 CB200, the blinker relay is under the seat, right behind the battery box, near the starter solenoid.
Mine, too, needs an absolutely top-notch battery to blink properly. Run the starter for a bit (if the engine doesn't happen to light off right away) and they won't flash until the battery is charged up a bit (ie, high revs...)
Generally, it happens due to 35 year old connections, especially ground connections.
All of the current for the blinker bulbs flows thru the relay first, before going up to the switch and out to the bulbs. So, poor connections, etc. will serve to limit the amount of overall current flow to the point where the old, original relay will not cycle, even though enough current is flowing to 'adequately' light the bulbs.
Clean ALL the connections (always a good idea, but time consuming) and/or get a relay that requires slightly less current to cycle.
Occasionally, bulbs of a lower wattage will get accidentally substituted along the way, which draw less current, making the problem worse.
Kirk
