Periodically I get the same from front side-lights. It is the connections where the connectors connect to the bulb holder on mine. I cleaned everything up using a mixture of salt and vinegar (baking soda in water should work too) but it still does it every so often. Everything is tired. Ideally I'd cut the lot out and solder in new holders, maybe for LED lights but that then throws the BECM into a wobbly unless you draw enough current in which case, why bother with LED? SO I go in circles and do nowt!
Right, spigot bearing today ...