Lennart, how did you get the knobs off the heater control?
The 3 plastic heater slide knobs are held by a 'pin' on the metal arm locking into a hole in the plastic. Actually it is not a very deep pin, but if the knobs are tight, as they should be, you need to open the knobs up slightly to pull them off. Some will simply pull off as they are either loose, already spread open or damaged. To open the back up is obviously difficult without the right tool, as you need to come back towards you with a fine tool poked through the heater slide holes. We mechanics have what are called 'pick' sets. These are small like a screwdriver, but the ends are pointed. The ends are variously shaped, but they have straight, 90 degree, 135 degree, etc. If you poke one with the point coming back at you into the back of the knob on the underside of the metal arm, and open it up slightly, it will pull off. If you don't open up a tight fitting knob, something will usually break!
If you have a knob off and you look inside the rectangular fitting hole, you will see a ramp moulded in the plastic so that as you push it on, it automatically opens up and slides over the pin, and then springs back down and locks it on.
The fan knob just pulls off, being a tight push fit with a 'D' shape for correct location.
As for the topic of extra fuses, just bear in mind that for every fuse you add at least four extra connection points, and no connection is perfect so you actually increase the circuit resistance as well as giving it more potential trouble points. You need to weigh the costs against the benefits in all respects.
Roy