The original style flasher unit/can works off the current-draw by the bulbs. The LEDs draw far less current, so the normal flasher does not work properly.
There are 6v LED flashers but they are not common (and I have not tested the ones I got from Sweden/Norway (motorcycle application, I think). Some people do a kludge/work-around by wiring-in a few incandescent bulbes under the body (to produce the necessary currect draw).
I'll see if I can find a currect flasher, and will edit this later.
Maybe something like this?
