Weird Tail light problem - a complete mystery

[BestHeadhunter]

I'm hoping for a miracle. My 2007 CLK Cabrio has a tail light problem. The lights all work, but they dance.
For 10 seconds they are normal, then they flick to the backup mode, all bulbs replaced with Bosch bulbs.
Inside, the error message has always been the same, even when the lights were fine; "left tail light back up bulb lit" or words to that effect.
The problem is that I have an MOT coming up and last year, it had the same problem but corrected itself in time for the MOT. This year, it's a nightmare.
Any ideas?
I've read a few forums and comments about bulb replacements, SAM units - but I'm really in the dark here.
Any help much appreciated

 

[ioweddie]

possibly earthing problem...have a look for loose or charred connections...or maybe your battery is on its way out

 

[grahamcol]

Sounds like bad earth to me.

 

[Oldspanners]

Had several weird problems a few years ago with lights and locking all disappeared with a new battery. But as above check all the earths.