If you've got one of these types of cigar lighters, then it's probably the issue. There's an inner and outer casing with a moulded-in screw thread so as you tighten it up it both earths and clamps itself to the trim panel. When they come unscrewed, they don't earth reliably and then start causing weird issues. Mine started with occasional loss of power at the lighter and developed into disco party lights when trying to indicate or brake and eventually loss of electricity when the car was left to its own devices.
I don't know how better to describe it.
Wonder if this is the case of the mystery drain on my daily BX. It seems to have a lighter like this which is temperamental to use. I might rip it out and replace it and see if this cures it. i=It needs a new one anyway.