time delay relay - turn signal lights

I have a 1976 ford with a 5th wheel tailgate.

I want to add 3 lights for each side of the tailgate for turn signals.  I want the lights to flash in sequence from the center of the tailgate out.

I had thought about getting a longer time delay main relay for under the dash, but then I'm confused about how to wire it in the back. 

I would like to think that I am somewhat knowledgeable about a setup like this, I guess I'm just getting "brain freeze" on this one.

Thanks for your help in advance.

Here's the circuit:

Note that VDD and GND for the ICs are not shown in the circuit drawing.