What Pete said.
I've not replaced this part myself, but I have a friend that rebuilds
salvage cars, and repairs BMWs as a side job. I was at his house one day and
the topic was a heater motor that had died. I said the ballast resistor was
the culprit, and watched him pull one from a heater system that he took out
of a wreck, and install it into the customer's car. I'd say the entire
operation took upwards of 5 minutes. Having said that, he had already
removed the kick plates and panels that were in the way. I do not recall any
screws holding the ballast resistor in place, but there could have been. My
recollection is that it had a connector (duh!) and some clips.
If you know which end of the screwdriver is the one you hold, you should be
able to replace this part yourself.