1970 B Body Ground Wires

Hi.I have a 1970 Plymouth GTX. It seems to have a bad ground for the lights on the passenger side. This includes the brake light and parking lights. My brother said it should have a ground one on the engine to fire wall. I cannot find a new ground cable or wire. Is there supposed to one on the firewall to the passenger side of the engine? Any help is appreciated.

Nope, body ground is a lead off negative battery cable to radiator support

On my 70 RR I had funny ground issues with the lights...
My solution was to attach a ground lead from the voltage regulator mount on the firewall back to the engine, I used 1 of the back bolts holding the valley pan.

Just my $0.02...

When I got the Sport Satellite back from paint . . . and I put together, I added ground wires to front end wiring harness for the lights, as well as in trunk for the taillights . . .no issues . . . lack of good ground causes lot of weird things as the current tries to get to ground . . .

grounds are your friends.
unless this is a 100% perfect show car, adding a few extra grounds will go far in preventing electrical maladies.