In thinking about it....I don't think the trunk grounds would impact your problem. The rear tail lights use the same ground and if those work, your bulb ground should be OK for the brake lights to work. Check the yellow wire for current when the brake pedal is depressed. Depressing the brake pedal causes both the brake lights on both sides to illuminate. In your case, the passenger side works, so the brake pedal switch is good. The green wire on the passenger side is getting current....that leaves the yellow wire. So, check the yellow wire for integrity....that wire travels from under the dash at the steering column...under the drivers side kick panel, through the drivers side wire tray under the sill plate...and back into the trunk. You could disconnect the trunk harness and check the yellow wire for current. That might help you isolate the problem.
As as Helter80 said, check the bulb sockets...but it's unlikely that both have the same problem in the same circuit. There is a splice in that yellow wire just before it spits from one feed wire to two wires, one for each brake light. You might check that splice for integrity.
