Here is a tip that works for me.  Maybe you already know it?

Anyway, I open the hood, take an electric hair dryer and point it into
the air cleaner intake and turn it on.  My 300 TDT starts up in really
cold weather, even without my block heater plugged in, when I blow the
hot air in the intake.

On my 300 D there is a plastic flex hose that comes off on one end
really easy and my hair dryer fits nice into that.

Paul Fretheim