back lights earthing out

My 205 GTI 1.6 has just failed its MOT on the rear lights earthing out ie.
when I put the indicators on the rear lights start to flash too (not
completely on and off but noticible). The offside is worse that the
nearside. I read in the Haynes manual that the offside rear lights are
connected through the nearside rear lights. I have cleaned all the bulb
holders and the earth plug which is under the felt trim on the nearside, but
the rear lights still blink. Does anyone have any ideas what I have missed
or what could be wrong?

Cheers in advance.

 

splice in an extra earth to each lamp and earth it to the body with a screw