I'm extrapolating from my 4/4 and guessing a bit, but I believe it would be a separate temperature sender, probably in the side of the engine block. I think the VDO instruments of that era were analogue jobs like the Smiths setup on my 1986 4/4. On my car there's a temperature sensor integral with the thermostat which feeds the ECU, but the ECU doesn't provide a signal out, it's just used for internal purposes like switching the fan.

This theory appears to be backed up by this: VDO Instruments Wiring Guide

But if as you say there's a wire leading nowhere from the back of the gauge and a blanked-off sensor location, it does indeed imply an ECU signal. But that would imply a wire from the gauge to the ECU. Here's the MEMS ECU pinout spec and you can see there's no send pin to a temp gauge

Last edited by Hamwich; 19/05/25 01:52 PM. Reason: added info
Tim H.
1986 4/4 VVTi Sport, 2002 LR Defender, 2022 Mini Cooper SE