The reason they replaced the wiring harness for the air bag for free was because it was a safety issue.
The inoperative heated seat is a luxury issue. They probably connected a scanner to the vehicle to check for body control codes and found that the Body Contol Module was defective which controls the heated seats. So, before they can figure out why the heated seat wasn’t working the BCM had to be replaced because that could have been the reason why the heated seat wasn’t working. Once the BCM was replaced then they were able to determine that heating element in the seat had failed because the BCM was communicating.
I don’t see how they did anything wrong.
The problem is, once an electrical component is removed from its’ package and installed it’s no longer returnable.
Tester