...running speaker wiring is impossible as the there is a sort of plug attached to the body of the car instead of just running the wires through the car body as standard on most cars.
Running new speaker cabling is possible.
The plug you mention is from the Delphi Sicma 1.5 Series. Now Aptiv.
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Part #s for reference: DTB5000FH and DTB5000MH.
It's a 50 pin connector with a number of "spare" positions available to:
1. Drill out a couple of adjoining positions - enabling a wide enough space to feed through new cables.
Down side is doing it properly and hoping you'll never need to remove the door.
2. Add new wiring to the connectors using the correct crimp terminals.
You'd need either Sicma-2 or Sicma-3 1.5mm male & female crimps.
Part #s 211CL2S2160 and 211CC2S2230