Rear Window Defogger problems of the 2004 Chevrolet Impala

Since vehicle was purchased in 12/04, rear window defroster performed very poorly. In 3/07 it ceased working, however dashboard control switch and I/p control relay appeared to remain functioning correctly. Troubleshooting revealed defective circuit breaker (mechanically broken armature) to be cause of complete failure. Following replacement of circuit breaker, I/p control relay, and antenna module, rear window defroster is back to performing poorly. Electrical grid appears to be functional - good heating pattern (when it heats) with no visually obvious defects. No obvious problems were found in I/p control relay and antenna module that were replaced. Electrical diagnosis indicates excessive current draw thru defroster grid, which causes circuit breaker to trip after several seconds of operation. Manually cycling the defroster control allows breaker to cool and can again be activated to provide 10-15 seconds of repeated operation. Note that electrically disconnecting the grid (at the window) does not cause the breaker to trip, indicating no short in the vehicle wiring harness. With possible exception of the oem circuit breaker, diagnosis revealed to obvious heat damage to any other components/wiring in the vehicle. Rear defroster grid appears to be source of the excessive electrical load in the circuit. Problem does not appear to be addressed in tsbs that I can locate to date. Discussions with several technicians has resulted in nothing more than guesses at this time.