where do 70 roadrunner taillights get there ground?

if i touch a ground wire to the taillight housing they work fine

They ground to the body so you might want to be sure you have a clean paint free mounting area. Or as you did create a ground wire of your own from the housing to the body.

Not sure how those ground but I would start with the 2 screws that hold the plastic cover on as they should go all the way thru to the metal sheetmetal/bumper (I ain't up on Roadrunners). Good visual on how the bulb case is connected to metal & an added hidden jumper works fine as needed

they make a replacement tail light socket with a tab to wire a ground on.
but otherwise as stumpy said.

newly painted and such always tends to cause problems with that.

i did see that little tab but did't want to use it. Where would i guy need to scrape the paint off to get the ground

I have never seen how a superbird is put together.

I would start touching that ground wire further away from the bulb till you stop getting lights, then see what holds the housing to that part of the frame.
scrape there.
it will be as robert says, some kind of screw or bolt holding the housing to the sheet metal, as everything else that touches should be against a gasket.

A quick and painless way is to make gouund wires running from the trunk latch bolt to the screw for the taillight cover. Now you have a good ground path for the tail lights, no mods to the car, no rusted bare metal spots.

This is exactly what I did the same day I bought my 70 Barracuda.

Mine is a Satellite and uses a pretty heavy foam gasket to seal to the tail panel. I think the gasket for a Road Runner is the same. I also had to run a ground wire. Near as I can figure they may have gotten their original ground from where the studs on the bezel penetrate the tail panel...maybe touching on the side of the studs?