Hdg failure - wiring issue
After loads of time try ing to debug, turned out to be a simple set of bad crimps. Suggest you get this checked out first - as an example, with the problem I had [which had been looked at during two weeks by 4 avionics shops including the Dimaond factory in Canada], in the end IAE at Cranfield had it sorted in less than an hour (where others had spent days changing AHRS, magnetometer etc.) as IAE started at the 'simple' level of just checking the wiring properly first.
It is hard to do as it involves contortions to get into the various places in the wiring loom (and there are 4 connections between the AHRS in the rear avionics rack and the magnetometer in the wing) so lots of avionics shops seem to feel it is easier simply to swap units out in the rear avionics rack rather than properly trying the basics of checking reliable continuity at the connectors and pulling on the wiring into the connectors to see if the wires are secure.