Trailer:

Q – attaches.
X decompresses supports.
Numpad 6 – Working lights (on the arrows).
Numpad 1 – to the left turn signal.
numpad 2 – Alarm.
Numpad 3 – the right turn signal.

Tractor:

Numpad 1 – to the left turn signal.
numpad 2 – Alarm.
Numpad 3 – the right turn signal.
numpad 6 / F twice – Working light.

Credits:

Royal Modding