OK So as I understand it the car has both the DB25 and 54 pin in the harness? That's a new one on me, usually the car has one or the other, and adaptors in beween.

Before sliding back the 54 pin harness as shown in the photo and checking the wiring colours against the list (you can see each pin is numbered and the wire colours are given in the form RT/SW which is a red wire with a Schwarz [black] stripe), try an see how the DB25 and 54pin coexist in the same harness.

Follow the loom and check to see if the wires are split near the wheel arch and available to both connectors or something. It could be that you have two looms and the 54 pin one isn't plugged in behind the dash perhaps, or perhaps both connectors are wired up. You really do have a setup I haven't seen before, and your car must have been built on the changeover point.

Do you have a multimeter? Is an essential tool and available from the market or Wilkinsons for a fiver. Get one tht has the extra set of sockets for measuring current.