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.