All those flashers go through a single 0.33 ohm, 5W resistor that is mounted by itself somewhere in the cabinet or under the PF. There is no circuit board.
If you look in your backbox, you may be able to find the one for the lightbox flashers. Its mounted on the backside of the lightbox, near one of the flasher sockets.
Attached is a snapshot of one in a backbox from Teed Off (in the left part of the yellow circle).
So all you need to do is disconnect that board, and get a 0.33 ohm, 5w resistor and mount it to the cabinet somewhere convenient. Make sure you get a POINT-33 ohm (that's 33/100 of an ohm -- ie, a fraction of an ohm -- not 33 ohms).