To add 4 way flashers you only need 3 diodes, a flasher and a switch. Find a constant power source to hook the switch to. I put them on my 65 Barracuda.
I'm sure google has a wiring diagram if you want to hook them up.
You can................Path is hot power source............fuse....switch....flasher...........diodes.......to signals
At the flasher, connect three diodes at the "arrow" marked end, with the cathode, the BANDED end toward the lamps. Three diodes. One to left front, one to right front, and one to the brake switch wire leading to the TS switch. I would use 5A or so diodes, 25V or 50V or higher PIV