Are you searching with a black light? The dye may go in red, or dark purple is more common, but it shows up as a bright yellow stain under the black light. Hissing in the dash indicates the evaporator is leaking. There is a condensate drain pan under it, and the water drains out behind the right front tire. Look under the hood, on the passenger side of the firewall, for a four inch long rubber hose with a ninety degree bend in it, hanging down. If you find dye inside that hose, that verifies the evaporator is leaking.
Leak sealers rarely work, but especially in an AC system, you do not want to add anything that will plug a leak. The heart of the system depends on a very small, carefully-controlled orifice, or valve, to control the flow of the refrigerant. The goal is to allow in just enough refrigerant that half is liquid and half is vapor. The point at which the liquid vaporizes is where it gets real cold. If your chemical blocks that controlled leak, the flow will not be correct. The cooling will end up taking place under the hood in the hose coming in or going out. That will not cool the air blowing into the car.
Because of these additives, we have machines now to identify when something has been added to AC systems. That prevents our equipment from becoming contaminated, which could render it inoperative, or it could contaminate the next car we use it on. Most reputable shops will not work on cars when they find that things were added. Your mechanic may come up with the same diagnosis as I did if you provide the same dandy observations and symptoms. During the evaporator replacement, he will probably want to flush the system to insure it works properly.
Be aware too that evaporators can spring a leak on any car, but one thing that can aggravate the problem is when refrigerant is added to a system that was completely empty. We always pump it into a vacuum for at least half an hour. In a vacuum, water boils at 77 degrees, and can be pulled out before the refrigerant is pumped in. If any water vapor is in the system, it combines with refrigerant to form an acid that attacks metal parts. That is a common cause of leaking evaporators and condensers. The water gets in as the humidity in the air when there is no refrigerant under pressure to keep that air out.
Wednesday, September 20th, 2017 AT 8:48 PM