Doh!
I started it.... my fault!
By rear side markers I meant tail lamp bulbs... I am still a little confused(Brit living in the USA) about the correct terminology!
Yes, there is only one per side in the rear, and I replaced them.
I replaced the front ones too and saturday I did the licence plate bulbs... still no joy.

All the bulbs are illuminated correctly now.
If that is the case I can only assume its the module.

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2009 Mercedes ML320 Bluetec