I bought this 66 Stout/80's pickup, and I have a mess of wires to deal with. Coming from front turn signal I have 2 green and a white. Going into fender I have this:

The second mess is going to be figuring out the trailer lights on the flatbed...

This is on the passenger side, the driver side is in the same boat. I was going to turn the signal on and see which wire had voltage but my flasher isn't ticking. Would the best option be to follow the wires back to the cab?

Find out what wires are the brake and tail light and the back up lights in the back the one left over will be the turn signal.

Your flasher won`t flash without bulbs (a load) in the circuit

it will be the only different colored wire left and right tail lights brake lights and reverse are all the same color left and right.

That wire will be the same color front and rear for your turn signals

Looks like you will get good a soldering if your not all ready.

Will the wires most likely be pairs? or will I have some groups of 3? Is it ok to wire all of the grounds together?

Yes the grounds can be tied together making sure of a good connection.

Not real sure just what you mean by pairs or 3 together

in the lighting circuit all the lights coming on with the headlight switch are most often the same color

With something that old who knows :jessica::jessica::jessica:

By pairs I just mean that each wire should have its own specific match. There shouldn't be any threesome going on...in theory...