How does one clean out the evaporator coil in a 1998 F-150? Is there a product that can be sprayed into the cowl vents for this?

I tried accessing the coil to clean it manually using directions from a Haynes book. That's a mistake! I got everything back together but could not get to the coil.

Also, are those stop leak refrigerants worth the extra $5? In my line of work I don't think I've seen one brand of R-22 or R-410 that's better than another. I'm thinking it's the same for R-134a...that they're all pretty much the same.