After replacing the GM3 and having it still not work, I had a much closer look at the wiring in the hatch, and at the hinge point at the top.
Found 3 frayed and one broken wire at the hinge point, after pushing (carefully) the grommet back into the hole in the roof to expose more of the wires.
Wires now reconnected. All works.
Anyone want to buy a just tested GM3?
TL
R - fault was in wiring by the hatch hinge.