I did mine last weekend without taking the spoiler off, Was a bit brutal and completely broke the old brake light.
First thing i did was use a trim tool to prize the brake light down away from the spoiler after a couple of cracking noises it fell out leaving the 2 captive nuts and screw to be slid out of the spoiler.
Then unplug the light and plug the new one in, Push the grommet back into the hole and job done for the wiring.
Now take some double sided trim tape an put 2 little pieces on the rear of the brake light, Push the remaining wiring into the spoiler and locate the brake light into position till you here it click into place. the brake light will hold in anyway on the clip but the trim tape is just to make sure.
Total price £34.79 and half an hours easy work.
Not bad compared to VW quote of around £300.