Ahhh I had lots of fun retro fitting an aux to my MFD2 (yes I know not an RCD510 but food for thought)
I did buy one of these adaptors to plug straight into the back but it didnt work first time around.
Turns out, that dependant of model/varient (to be honest many different things) inc various settings within the head unit itself.
The quadlock connector at the back didnt make the blindest bit of difference, was more of less just a hole
I ended up cutting the quadlock connector off, looking at the wiring diagram at the back of the head unit to locate where L IN, R IN and GND was situated, added the 3x cables from the quad block to pre-existing slots on the loom (no cutting involved)
Pressed Aux, tested = Job done !
I did however buy a headphone pre-amp for £25 off fleebay as the audio output form external sources was a bit low
I can send some pics if you run into trouble, as I took them along the way just in case
P.s This took a lot of research and phone calls to realise this, should have just looked at the wiring diagram in the first place