No heat in 1990 500sl , one minute working - next its not. Want to replace the inexpensive $85.00 acc mono valve before i replace the $500.00 climate control unit. Dont know where to look for the mono valve on the 500sl. have come across instructions on just about every other Mercedes model but the 500sl . Any help would be apprectated. Heater hoses are buried cant locate. I'm sure it somewhere near firewall ????

If your car was manufactured in 1990, you ought to run diagnostics on the controller as explained here. The monovalve is shown below.

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