The easiest way would be to sacrifice the low speed switch, change the blower resistor for that speed to a smaller value and then the fan would run faster when you select that speed. You;d have to experiment with different resistor values to get the one you want, or just put a wirewound pot in place of the resistor, and when you get the speed you like, measure the resistance of the pot and then replace the old resistor with that value.

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Theo

1985 300D turbo 235K miles, Delvac 1, mostly B20 BioDiesel

klakka klakka klakka klakka...Arf arf arf arf...
How does that dog always know when Theo's home??!