Copied off another forum hope this helps.Ive just fitted a RNS 510 into my MK5 R32. The radio antenna adapter i used was from VW and the part number is7L6-051-551 Genuine VW part cost me £22.50 i ordered mine and collected next day.
Have a look here matey post #21http://vwnavi.com/forums/showthread.php?p=39776