Ernie,

If you are smelling refrigerant out the vents, it is likely the evaporator. Red dye doesn't do much good. It is difficult to see. You need to use UV dye and then look for it with yellow glasses and a black light.

Once the system has the UV dye and is cooling well enough to make condensation drain water, shine the UV light in the drain water and look for the dye. If it is there, then the evaporator is the culprit.

I was very lucky because I found lots of UV dye at the expansion valve. As psfred said, the expansion valve is on the firewall in the engine compartment next to and behind the brake booster. Just follow the lines.

I have had the dash off almost all the way to the evaporator. It is a time consuming job, but not complicated as long as you have the printouts of the appropriate sections of the MB CD manual. Most of the various fasteners that you deal with are not difficult. While you are in there, you should replace ALL the vacuum pods and the rubber elbows that connect vac lines to them.

If you were to get the CD you can work through this. If you get stuck there are several people here that can help you past the hurdles. The pods and the evaporator will total to about $500 in parts if you buy them from fastlane here on the site. Unless it's been replaced lately, you should probably replace the filter/drier which is about $42. R12 is coming down in cost and can be had for under $15 per pound. I just bought a 30 pound tank for $425 total cost delivered to my door. It might can even be bought cheaper than that.

Good luck,