Unfotunately it can be any where along the circuit. You are going to have to trace the circuit back from the light and see if you can discover what is going on.

When I had a similar issue on a old model US Car I ran a seperate wire from the headlight (in your case the low beam) that always worked to the head light that did not work even though the headlight was fully functional.

More recently on my Chevy Astro Van the low beam on the Driverside was always dimmer then the passanger side and swapping the Headlights did nothing.

I had something bust inside of the Head Light Switch. After I replaced the Switch the drivers side Headlight shined as it was supposed to.

On other lights I have had issues with the light sockets or the connectors.

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84 300D, 82 Volvo 244Gl Diesel