I have heat only when the the fan is in LO mode, obviously don't care now, but in winter takes a while to get the dew off the windshield.

My tech says is isn't the monovalve, but more likely something electrical, around $250. Is that the push button unit? Why would it only work when fan in lo? weird? This is for the 300.

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'89 420 SEL
'90 300 SEL
'68 Olds 88 Convertible
'84 300 SD (sold it)