If you replaced original incandescent bulbs with L.E.D. bulbs, It could be that the impedance is insufficient to cause the flasher relay to cycle on and off as designed. Generally speaking, a quick flash indicates a failed bulb or other disruption of continuity. An easy test for that would be to put the original bulbs back in and see if the issue resolves itself. Turn signal bulbs ground thru the socket sleeves. To confirm the ground path of the turn signal sockets, use a volt/ohm meter set to ohms and touch one of the leads to the socket sleeve and the other end to the negative battery post. A disrupted ground path would read as open or display no numeric value. If you have pulled the negative battery cable, test to the transmission case where the battery cable fits up. 

• You have most likely substituted bulbs of the wrong internal resistance or voltage. There are numerous 12V bulbs with a single filament, but only 1 bulb number will function properly as a Turn Signal. This is because the actual flasher module uses a "bi-metal component" that is counting on a very specific current. As electrical power flows through the bi-metal arm it heats up. As the arm heats up, it starts to bend and flex away from making electrical contact. And just like a manual switch, when the electrical contact is broken the light goes OFF. The bi-metal arm (now with no electrical current flowing) starts to cool, and so it bends back to its original position, once again making contact, and the process starts all over. (Watch this YT video: https://www.youtube.com/watch?v=f17dXzrpl90 at about 4:00 to see this in action.) Those very specific bulbs are what determines the very specific current flow to trigger the bi-metal flasher operation.

• If the bulbs are lighting up at all, then you have some type of "ground" return. Inside the tail lamp housing there are 3 Brown return wires that mate with the harness (L turn signal, R turn signal and the TL itself). Unplug those Brown wires and treat the male ends with a compound that fights corrosion, such as No-Ox-Id by Sanchem. 

• Turn signals don't stay ON long enough, to get hot enough, to chase out the humidity. What I find very often is that the 50 year-old socket for the bulb (the steel part that contacts the brass "base") has some rust on it. That rust may be adding electrical resistance to the flasher circuit. These rusty sockets can be treated with, you guessed it, No-Ox-Id by Sanchem. 

Hope this helps.