Making your non-flashing corner markers flash is easier than you think. By cutting one wire and adding a jumper, you can make your marker light flash alternately with your turn signal.
The way it works is this; you cut the ground wire to the marker light, and splice it into the turn signal's power (flashing) wire. It then grounds through the turn signal lamp. Now, every time the turn signal lamp energizes, it cancels the marker light. When it de-energizes, the marker light grounds through the turn lamp. And so on, and so on.
Here you can see the ground wire cut, and a jumper connected. No need to do anything more with the cut end of the ground wire, because it is just a ground wire.
Here you can see the jumper spliced into the turn signal wire. The ground wire and marker light wire are left alone.
Here they are in action:
Protecct and fasten the wires as needed, and that is all there is to it.