Need a little bit of wiring assistance

I have a 70 Duster that is just giving me fits with some of the electrical. Simply put, I am not an electrician and I am trying to figure this out....

When I pull the light switch out, I get headlights, but no tail lights or dash lights. If I turn the knob, the dome doesn't come on either. If I press the brakes, the brake lights come on, passenger side dimmer though. If I put it into reverse, no back up lights. Before I take the dash apart, would a lose ground wire in the trunk be causing all of this? I'm looking for the ground on the tail light assembly, but can't find hour it. It has to be there somewhere....

Any advice is greatly appreciated