Problem finally solved.
Bought another M-Ask2 unit thinking the first was faulty, same issue present.

Decided the only culprit could be the MOST loop. Despite conducting light all the way without breaks, there must have been attenuation somewhere that that the champ can deal with but the m-ask2 cannot.

I temporarily ran a new 5m fibre direct from headunit to the TCU and it all worked.

In the process of installing the new cable now, back seat has to come out to finish the process.

Order has been restored to the universe.