I put nearly 70,000 miles on a '97 3500 auto with a PacBrake using a "mystery switch" towing a heavy 5th wheel. The biggest disadvantage is that it will not lock up the torque converter only hold it locked. Also it only works on third gear or overdrive and you cannot shift down with the TC locked. If in third gear with the PacBrake is on, and the TC is locked, and you are going too fast, you must unlock the TC, shift manually to second gear, and with the exhaust brake on, the transmission heats up rapidly.
If you want an exhaust brake setup on the automatic transmission that works like it should, call Bill K.
Bill