Symbol mystery roof leak

This winter our van (now 10 years old) has developed a leak.  Water drips out of the 12V cigarette lighter socket, which is just behind and just above the sliding door.  So leaks through the door seal can be ruled out I think.  The van is parked sloping front down, so once again this would seem to rule out the possibility water is getting in through the door seal.
Taking off the socket, and the electrics control panel, I see that the socket is slightly wet - but no sign of anything else.  No water tracking down any of the cables, no sign of water anywhere.
Has anyone else seen this problem, or have any suggestions where to look?
Note: I did think the roof TV aerial might be the source, so took that off a few weeks ago and resealed.  So not there...
Thanks!

Awning light / wind out awning brackets near by ? Could it possibly be one of these ?

Thanks Chris,
Not the light I think, that's some way forward and the van slopes quite distinctly nose down.
The middle awning bracket looks in about the right place.  Doesn't look easy to take the awning off though... I  will see if there is something I can use to temporarily seal around the bracket - but even that doesn't look easy to get to.
  
Angus

If in doubt, always try (and this really isn't a wind up!) 
CAPTAIN TOLLEY'S CREEPING CRACK CURE.
Buy it online.
It does what it says, and permeates into any crack by itself then sets in there.
I have fixed 2 window leaks on  my van with it now.
Be certain NOT to get any on paintwork, mask the area where it may run down.

If the awning has the purpose made brackets for it it may not be leaking from there if they installed it correctly as the brackets fit to lugs on the roof top , somebody used a endescope the other day to chase a leak , perhaps you can get your hands on one to have a look for any tell tale signs of water ingress

You are correct about the awning - I forgot - the awning is on the factory fitted lugs, so there shouldn't be a possibility to leak.  HOWEVER, there are some pointless roof rails on the roof, fitted by AS, and potentially there are holes in the roof for that.  The middle fixing is close by.  I can easily re-seal that.
BTW I say pointless, because I asked AS what the safe load was for the rails.  They said "zero, just for decoration".  A bit bizarre as you can't really see them!
I do have an endoscope, I hadn't really thought about getting that out as I can see and feel the problem area.  See photo.  I think I'll probably wait for the next lot of wet weather, take another look and if I can't see anything, get out the endoscope.
@IanH, thanks, I'll look into that.
Angus

Yes the decorative  roof rails are fastened on by screws so possibly the source of the leak

While your in there check how much proper insulation is visible. Could just be a build up of condensation ( hot cold hot cold weather) finding it's way out. Good luck anyway.

While reading the thread before I got to your reply babian, I was also thinking possible condensation.
I was delving into the darkest corners of my van running wires this time last year and the amount of hidden condensation was quite shocking.

Hi, I have a Dorset ES which is on the Boxer Chassis and have a mysterious leak dripping down inside the 'B' Pillar and onto the passenger seat belt drum. I completely taped off the sliding door and the awning light. The water still drips in but only when it rains. It seems to be coming in above the level of the sliding door. These vans have a fiberglass roof (cap) fitted above the existing van roof.