Should be pretty simple to check with a multi meter. With engine running and AC on, do you have 12V at the switch (no - bad switch, low refrigerant, etc.). If you have 12V at the switch, check the fan. If you have 12V at the fan and it's not running, replace the fan.

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Charles
1983 300D, bought new, 215k+ miles, donated to Purple Hearts veterans charity but I have parts for sale: http://www.peachparts.com/shopforum/showthread.php?t=296386