vwnat

Hi all I have a 99 c43 AMG im puting my kenwood x-592 in it I got a metra 70-1786 amp int harness put it together ( all no prob) deck runs plays etc but no sound at all ( yes the rca are pluged to the right place and I double checked I have a factory amp). at the back of the factory deck there is two plugs the standard vw type plugs and a thiner wider one above it has three very small gauge wire off it and it says AMP right on the plug , this plug does not plug into my adaptor harness , but no where can I find anything that says I need this ? I suspect this is the problem or the keenwood has 4v pre outs to much for the factory amp? any help much apreciated thanks Nat .