Back to what Larry said about adding dye.

The drain water will likely contain traces of this dye if the evaporator is leaking. That will be easy to get to. It will be laying on the ground under your car. Park car on concrete garage floor after having added dye and run for awhile. Back car out of garage after you have a puddle underneath and use special light to examine drain water. Use the search facility for the type of light used to locate dye.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"