Installation “Ambiente lighting”

Moin together, maybe it’s a stupid idea: I would like to put an LED in the front ceiling light as “ambiente lighting”. How can I wire it so that it can either a) reach through the glare light switch or b) when the ignition is switched on. But I don’t want to dismantle the entire front interior covering to move 20 m of cable. So: Is there a possibility of wiring that takes place in the immediate vicinity of the ceiling light? Maybe someone has already tinkered with it and can describe to me which of the wires lying there I should connect with the lamp. Thanks already in advance!