Don't know much about 190s but this is how ite works on a 124. Power is fed to both mirrors whenever the ignition is on. There is a thermo switch on the back of each mirror glass and the element is bonded to the glass.

So check for power at the mirrors when ignition is on, if there is then either the element or thermo switch are faulty and you need to replace the glass.

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Mick J
'08 Chrysler 300CRD (MB OM 642 engine)
'95 E220 estate
'89 230TE (R.I.P.)