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 ????