306 Rear Lights Problem

[deere3350]

Hi all,

The rear lights on my 306td are currently giving me a bit of grief. Everytime the indicators are used and the lights are on, they go dim in time with the indicators. I understand this is caused by a bad earth, but don't know where to look. I can't get my head round the Haynes wiring diagram, either.

Anyone have any ideas where I should start?

Thanks in advance.