Right, First things first. There is no algorythm which links the security code to the serial number of the stereo, so it's a bit more involving, although if you've got the right tools or know someone who has, it's not too difficult a task.
You will need:
Slotted screwdriver or table knife
T8 Torx screwdriver
an old PIC programmer, these are about £6-8
and depending on how you want to do it, either a test clip, or a soldering iron&solder.
You will also need to download Ponyprog and Grundig WKC series Code Calculator v4.00
First, prise the top casing of the head unit off starting from the back corners, with the screwdriver/ table knife. You can then lift up and pull it right the way off.
You then need to unscrew the CD mechanism, (4x T8 Torx screws)
Once unscrewed, you can lift the CD mech up out of the way. I have highlighted the chip we're interested in (24LC16)
The chip highlighted above is the EEPROM and you need to create a dump of the data on this chip. You can either desolder it and insert directly into the PIC programmer (very fiddly) or you can get a test clip and wire it into the PIC programmer so that you can plug the test clip straight over the chip whilst still on the PCB
I'm still waiting for my test clip to arrive, so here's a stock image, so you know what I'm talking about.
So you open up ponyprog, calibrate it for your programmer and Read the EEPROM. Once you have read this, you need to go to Line 000010 and take value 1,2,5 and 6, as I ahev highlighted in the image below:
You then take these values, open up the Grundig Calulator mentioned at the top of this thread, set the model to 'Seat Aura' from the list on the right hand side and enter the 4 values.
Then click 'Repair my dump' - It fills in the remainder, all boxes turn green and your code is generated.
Put your stereo back together, put it in the car and enter the code. All done