Maxwedge: Today I applied mapp gas to the 12 mm nut for about a minute (just long enough to bring the nut to a temperature below the point when it would glow red); then with vice grips holding the square rod portion of the cable and a 12 mm open end wrench, the nut easily broke loose. I let the nut cool and applied pb blaster; then worked the adjusting nut loose and spun it counter clockwise to tighten the brake cable. The parking brake cable is now tight again and when the parking brake cable pedal is depressed, the rear two parking brakes once again hold the vehicle until the release lever is pulled. Thank you for sharing your advice regarding heating the nut to break it free. It worked like a charm.