Potential serious issue with car I have just purchased. Thoughts on rejecting it??

[JMR1]

The two rear lamps were the obvious places, but it could getting in anywhere around the glass too. If you open the boot, so the inside of the bootlid /boot door glass is visible, there should be a cosmetic rubber or plastic seal running all round the frame. This should come off easily enough ( and go back on as well )Gently pry this beading off, close and lock the door, then using a garden hose, allow a gentle ( gentle is the operative word here, when dealing with water leaks. High pressure just seems to drive the water over the leak ) Fold up a towel on the roof, near to the gap between the roof and rear door aperture, and using something heavy to keep the hose in place, allow the water to flow down over all the door. Get into the back seat and watch what is happening. if its a bad leak, you wont have to wait long, but a small leak could take some time to make itself known. As a preventative, before replacing seal for the glass, put a thin bead of silicone all round where the rubbber fits. The excess will be squeezed out when the beading is replaced, and can be cleaned off then.
It could also be along the rubber seal the door rests on, or any of the welded joints along there. Is there a sunroof on the car?

[Tinytimbo]

Quick update. I replaced the light seals a while back and I'm happy to report no more water is getting in after rain fall.

glad I didn't reject the car as I'm really enjoying it now. All my previous cars have been VW and this A6 really does feel a step up.