Relays work by having two separate circuits. One is the "energize" circuit that closes the relay, and then the other circuit takes power from another source, usually a fused battery source, and this secondary circuit is the one that actually sends the power to the item being fed current. Lights for instance. The light switch sends energizing current to the light relay, which closes it. Then the battery supplies current through a fuse on a separate line, to the lights. This makes the light work. Taking the current directly from the light switch would burn out the switch, so a relay is used. Maybe your relay is wired wrong. With the light getting its power from the door switch, maybe the relay is wired into that wire somehow. I don't think you need a relay for the interior lights. So somehow, a wire got put into the relay and is being fed by the interior light, when the bulb is on. The bulb completes the circuit, allowing current to flow, and this then is flowing through the relay, for some odd reason. The relay may not have any thing to feed, so maybe it just then clicks on and off. I don't know. It is strange.