I have just diagnosed this on my dads 400. Exactly the same fault..speeds 1 and 2 not working.
If you drop the glovebox by squeezing the sides so it lowers more than usual there is a small panel with a four way plug in front of you (wires are all blue + coloured tracer). it is held in with two screws. It has three resistor coils on the inside (in air flow to keep cool) like bulb filaments. The larger coil will be broken. I'm still to source a replacement so can't help you there yet.
Andy P
December 2008