Rear Sidelight Bulbs going black
 

Hi. Can anyone help with this problem? The rear sidelight bulbs on my VW Golf Mk 4 go black after a short period of use. The blackness is on the inside of the bulb glass. I have replaced the bulbs a few times with ones from Halfords, etc. Unfortunately as the bulb gets blacker the light gets dimmer and I keep getting flashed because people think I haven't got my lights on. Any ideas? Thanks. Jonty