2012 Mustang GT help

Hello everyone, I am new to this website I just need some help with my Mustang. Over a year ago, I took my car to a car audio shop to upgrade the sound system. They added an aftermarket radio and a Metra kit for the HVAC controls. After a few months, the Metra screen would turn off and the defroster would come On full blast. I took it several times to the shop: they would tell me it was defective and that they would order another one, which they never did; they told me to remove one of the fuses (they told me which one) and put it back in and it would reset the system. I did that a few times and it would work. Just last week, I was driving with the AC on and it suddenly shut off but the AC controls were still ON. I turned them off and then back on and I would not get any air coming out of the vents. I thought it was the Kit that they had installed so I took it to another car audio shop. They told me it was indeed defective so they sold me another one. In the process of getting it installed, the technician told me that the AC doesn't work with either kit, they even tried it with the original dash kit and the result was the same. I took it to a mechanic shop and they checked everything and told me everything was fine, it had to do with the wiring that they had done when I had the install done. The new shop is stating that they did everything right and it was probably some mistake from the previous shop. I made an appointment with the second shop so they can check the wiring. Can you guys help? Thanks in advance.