Make sure rain water is not accumulating in your cowl vents. In my W123, the outlets on the firewall were clogged allowing water to backup into the ducting, soaking the passenger floorboards and ruining an expensive blower motor. Not sure if your W126 has a similar layout but worth looking into.

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Codifex
1981 240D ChinaBlue (Got her running with a donor engine.)
1983 300DTurbo w/sunroof.
1984 300TD manual sunroof. (Electrical Gremlins)