I bypassed the servo coolant lines on a friends rusty 1980 300D. His servo was completely shot but all he wanted was heat/defrost. I installed a $16, cable operated heater valve (for an '80s Dodge Omni!) on the firewall, with a lawnmower control fastened under the drivers dash. The original 300D system had a default mode to operate the defroster which was still functioning so my friend was very happy.
Very Happy Motoring, Mark