Sorry didn't know how else to describe the problem (which may be why searching hasn't helped

So when I turn the ignition half way (electrics etc) I hear and feel a noticeable current of air coming from the pillar behind the drivers side head rest around the point where it meets the roof. This carries on when the car is started and when running.
The van is a 97/98 model and has an aftermarket "sky lift" roof which was installed by JAL
I haven't had any panels off to investigate further yet (partly as I don't know how to without risking damaging anything)
Any advice on how to proceed would be greatly appreciated.
Cheers
Dave