I have a similar problem with my RNS510 version H - absolutely no AM reception. When fitting, Eddie-NL checked the Fakra connector, and swapped the leads over - still no AM reception.
He tried another RNS510 unit (Skoda unit), and no problem with the AM reception - meaning my unit is at fault, not the aerial circuit (I guess).
Not sure of the inner workings of an RNS as to why a unit would fail on AM vs. FM - I'm not too fussed, as I don't listen to AM, but don't like things not working properly!
Bump for the OP - any info gratefully received!