The problem with dye checking under the dash is that you have very little visibility. The best way to check for a leak in that location is with a refrigerant sniffer. Without that, you can't get a light in there and shine it on the entire assembly unless you removed it. Sometimes if the leak is a bad one you can find dye traces on the evap case drain hose outlet.
If the symptom is indeed a low/no refrigerant charge, you can pinpoint it through process of elimination. Everything else is easy enough to check with dye and a light under the hood, so if nothing else is leaking it has to be the evaporator.
Make sure when you do dye testing that you are also charging the system along with the dye injection, then you have to run it. The dye mixes with the oil charge, so if it is not run the refrigerant will leak out without the dye circulating to show the leakpoint(s). Depending on the size of the leak you may have to run it anywhere from 30 minutes to a week or two. Some leaks are also only pronounced when it is hot or cold out, which can be difficult to find.