Minor electrical mystery

When I removed the bed on my 86 B2000 I cut out the taillight and trailer wiring mess left by the previous owner.

I cut the 86's taillight wiring back to where it's grounded to the frame and rewired using the 87's wiring which I know worked when I had it on the donot truck.

Everything works fine except for 1 thing (this is where the minor electrical mystery comes in)...

When I use the turn signal arm to turn on the lefthand turn signal, it will go on but not blink on and off. When I use the 4-way flashers switch, the lefthand turn signal will blink on and off.

It's obvious I'm getting power to the turn signal but why won't it blink?

Any suggestions/ideas as to why this is?

Cheers !!