Jason, I have been doing a bit of digging and it looks as if you are going to have to bite the bullet and get to the rear of the instrument panel. I don`t know how different it is to the GTA`s, but someone on here might be able to point you in the right direction.
Now, I can speak a bit of French, but when it comes to technical jargon.....I am not so good, so....if Andy Holt can confirm that "info. debimetre" with a couple of accents on the first and second "e`s", is, in fact, the Fuel Flow info from the ECU, then read on. Mind you I am pretty sure.....but you never can tell
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Pic 1, is of the circuit showing the path....in Green... of the flow info to the computer. You may have to print it and turn it on its side for future ref.
Pic 2 Top. shows the dash (LHD) and position of connectors R173 and 109. I doubt if R173 is giving any problem, but for clarity, I show it.
Pic 2 Bottom. shows all the connections to the Instrument panels board of connector 109, and pin 5 (381) with green wire is where the Flow signal enters the board which in turn then goes to the computer. I don`t know how it gets to the computer as there is no detail, and I have not seen one. But either the computer is soldered directly on to the board, or there is a multicore cable leaving the board and terminating in another small connector going to the computer.
Pic 3, just for info of R173 showing that pin B3 carries the signal through the connector.
If you do go down this path, once the instrument panel is out, could you take a couple of pics of it for me, as I would like to see its construction.
Good luck
1990 GTA Atmo, 2003 Jaguar X type 2.5SE Auto, 2018 Kia Picanto GT-Line-S 1.25