Just some general information that I gathered on upgrading 3C Passat Head Unit's to a RCD510/RNS510
VW Passats built before 2009 were not designed to take the RCD510 RNS510 RCD310 etc head units. The issue is the Passat 3C gateway 3c0 475 530 C firmware version 40.
When you plug the unit into your car, you will be able to here a clicking noise coming from the passenger side dash board, it is a relay switching on and off and is causing the battery to drain because your current gateway is not compatible with the head unit.
The easiest solution is to replace your can gateway with a revision N and above model.
There is not a lot of information on the Chinese RCD510 units that are easily found on eBay China. Model 5ND 035 195. These too will cause the battery issue. The only models that won't are very early revision RNS510's that have the old software, as soon as they are upgraded they will fail.
Please note that you should NOT use a gateway from another model car, because they cannot interact with the electronic column lock in the Passat which will throw an error.
If you know somebody with the right equipment, you can upgrade your gateway to version 048 which doesn't exhibit the issue. Do this with a VAS5051a and the latest VAG flash disk.
Last, you can do a "hack" fix, that essentially removes power to the head unit when you turn off the car, this requires some soldering in your fuse box. However if you do this, you will lose any settings every time you power off.
Leave a comment if you are in need of some more information, regarding this or anything golf/passat related.