To say that the heater box has a magic seal and once the magic seal is broken all is lost? Bull.

You could replace your evaporator yourself. Parts including replacing some bad pods, exp valve and drier would be less than $500.

Do a search. Lots of us have done this job.

Assuming a leaking evaporator is your problem and it looks like the stealer thinks it is, since he wants to replace the box and that's the only part in there that would cause your AC not to cool at all.

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2012 E350
2006 Callaway SC560