Sounds like an earthing problem to me. Does your interior light work, heated screen, indicators, number plate light? There is a common earth wire to most things in the rear of a 260 usually black, if you have a meter check its resistance to a good body earth from one of its connectors (rear light connectors are usually the easiest to get to for this). If this is very tiny, check the rear light connector blocks for corrosion and clean were necessary.
Hope that's of some help,
Steve