ok so the saga continues.
Disconnecting the glow plug module produces the P0670 code twice. As well as the expected other codes (loss of signal etc).
Decided to check the wiring harness by finding a wiring diagram from the dealer. They didnt have the pin numbers, but they had wire colors/size, so found the wires connected to the EEC by checking with the continuity meter.
The continuity meter gave normal sound, but on a hunch decided to check resistance. For the 3 main wires (module power, ground, LIN signal):
30ohm, 0.7ohm, 0.7ohm. So bingo.

To be sure I connected the glow plug module directly to the EEC, and the P0670 codes disappeared. (Note 15 other codes appeared but they're just related to the disconnected sensors on that same cable harness)

For anyone else doing it, it's the red with yellow-trace (red and yellow stripe) wire on the EEC plug (3rd connector from the right side I think). And on the glow plug module connector it's pin #5 (5th from left, top row. There's also tiny numbers inside the plug)

Next saga is how to replace this single frickin wire at the back of the engine without paying for a new wire harness.

Anybody else have to do that? Did you just add a byass wire, or did you repair the cable harness by removing?

Attached Images