Just to offer a little perspective, drivability problems and electrical/AC problems can be frustrating and time consuming to diagnose and fix. Often a technician can spend hours finding out that the offending part costs only a few dollars. Customers are happy to buy the part and pay for installing it but not so happy to pay for all the time figuring out what needs to be replaced. The alternative is to just make an educated guess and replace the part most likely to be causing the problem. Sometimes this doesn't do it and the customer is again unhappy. Underdash work is also very physically and mentally demanding.