I replaced my Challenger dash harness with a new one from M&H but I really didn't need to. After setting the two harnesses side by side, I realized that the original harness wasn't in bad shape. The main dash harness is the most expensive harness on the car.
On my Dart, I cleaned up the original harness before I made the decision on whether to replace it or not. It looked really bad and mice had gotten to a few wires. Since the Dart is a convertible, the harness was exposed to the elements and looked a lot worse then the Challenger's. However, after all the wires were cleaned up, it didn't look that bad. I replaced a few wires that were chewed up and put new ends on others. Then I tested for continuity on both ends of the harness. The harness turned out nice and I saved $500+.
If you do the alternator bypass wire and relay modifications, you'll take a lot of the load off the main dash harness. Unless the original harness is all cut up from previous owners, I'd save the money and clean up what you have.