Hello paolo,
For this you will need a relay with two relay contacts, one contact for the left-hand headlamp lowbeam circuit and one contact for the right-hand headlamp lowbeam circuit because the headlamps each have their own fuse.
The wires for the headlamps come directly from the headlamp stalk switch on the steering column and the IGN wire which is used to power the relay when the ignition is in the ON position only (do not want lights on for ACC or Start) will come from the ignition switch. These mean that the best place to access all the required wires is in the steering column, probably at the base where the wiring harness spreads out to the rest of the car.
The first diagram shows where the two relay contacts are wired into the headlamp circuits. Note the wire colours.
The second diagram shows where the relay coil is wired into the ignition ON circuit. Note the wire colour. You will have to find an earth point nearby to wire the negative side of the relay coil to.
The third diagram shows the positions of the two connectors in the steering column and shows how the wiring joins together into the single wiring loom which exits at the bottom of the steering column.
Wiring the relay in this configuration will ALWAYS have the lowbeam headlamps come on when the ignition switch is in the ON position only and still allow the lowbeam and highbeam headlamps to be switched on with the steering stalk control as normal. You will not have a way to switch the lowbeam headlamps off when the ignition switch is in the ON position. From your question in the initial post it seems as though this is how you wish it to work.
