https://forum.ih8mud.com/threads/fast-blinker-problem-on-78-fj40.266572/
That’s the thread I jumped into a while back, maybe some of the replies might trigger an idea for you
I have the same issue albeit from a different starting point on my 78 hj45.
I initially had turn signal issues. Intermittent and temperamental.
Replaced front light units, grounds, bulbs, fuses and eventually replaced the flasher unit with current Toyota solid state relay and taadaa found...