A/C hiss behind dash

Hi,

More than likely this is the evaporator that is inside the HVAC housing. The evaporator is the part of the AC system that has the Freon go through it and then the air blows over it before it comes into the cabin.

There are two ways to try to confirm this. First, put more Freon in it so that you hear the hissing and use a "sniffer" to determine if there is Freon coming in. If so, then you need to take it apart to find the leak. The other way is to put system dye (it may already have it) in the system then take apart the dash and use an ultraviolet light to find where the dye is and that will tell you what is leaking.

Again, this is pretty sure to be the evaporator because it is located just behind the glove box in the HVAC housing.

Using a good quality borescope so you can see into the HVAC box. The next easiest thing is to remove the blower motor which means you probably just have to drop the glove box. Once the blower motor is out, you will have direct access to the HVAC box however, you still may need some good lighting and mirror to see if there is a leak.