wire harness burnt under floor/rear seat

We replaced the muffler with an aftermarket muffler at 104000. miles. Soon after, 3 weeks, electrical problems started. Passenger air bag light came on. Passenger side window would not go up or down. Dome lights went out. Radio began to crackle. Dealer investigated. Found wires in the back floor melted and carpet burned. The car is not drivable due to safety reasons. It has been at the dealers for a month now. No help from Jeep. For us to put 4500. into this jeep at 106000 miles seems to be not feasible. Has anyone had the similar problems? Thank you