This might sound a little strange, but remove the tail lights and reinstall them. The tail light housings ground thru the six little nuts and bolts that hold them in place. When they are out, you can sand down inside the trunk where the bolts ground against the body. If you look in the center of the rearward facing side of the tail light housing, you will see a little male spade. It was built into the unit to run a ground wire to, however, the factory saw fit to never install one. I run a little pig tail down to the screw hole under the tail light that the cardboard cover is fastened with. When the unit's grounds start to corrode, they can only ground one bulb's worth of electricity. When you ask it to ground two, it can't handle the load. I've seen all the lights go off when the directionals are turned on, or when the brake pedal is pushed, or when the radio is turned on. These cars do have some funky ground issues, and they like to be driven every day.