A good scanner can run each component separately. Eg blower at 50%, each flap motor (feet/dash/defrost). I would suspect the porcupine or the part you replaced (or wiring to it).

If only blows the fuse when demanding heat the logical defect is the hot water valve. Unplug the power to that and see if the fuse still blows (or measure Ω to it)

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)