Who knows about turn signal wiring?

If you're using a column from a mid-seventies Imperial, I would think the turn signal switch would have the same colored wires as in the diagram I posted.

You will have to find the wire in the column (probably white) that feeds the brake light power to the turn signal switch. From your description of the turn signals working, it sounds like you already have turn signal flasher power routed to the turn signal switch.

Remember you will have to completely disconnect the brake light switch power to the brake lights as you have them wired now. If you can find that (white) wire in the column, route brake light power to it, then just let the turn signal switch do its magic and you should have it working.

It's easier to picture in my mind than it is to explain it on paper! Keep us updated!