merc clk320 reverse lamp failure

have a clk cab 320, as soon as i switch on thre ignition it comes up lamp failure. the reversing lights do not operate and the wingmirrors will not tilt down. very occasionally when i start i don't get this message and
every thing works fine. just had it in the dealer, and they cleared the fault codes and it worked fine for a day. then half way home it beeped again. needles to say it no longer works again. i'm a mechanic and have checked every connection i can find, and nothing makes a difference except clearing the fault codes. can anyone help? snipped-for-privacy@hotmail.com cheers
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