1984 500 sel cabin exhaust fumes

[paulpic]

Hi All out there.
I recentley bought an immaculate 1984 500 sel w126 (138 000 klms) with some minor issues to repair (high idle,air con needs regassing etc).
One issue i have which is a bit of a concern is that i get 25 parts per million of carbon monoxide in the cabin when driving.This is considerably worse when the sunroof or drivers windows are open and rear windows are shut.Its not so bad when all windows are opened but still noticable.
I have placed a fog machine up the exhaust and get no leaks in the mufflers/exhaust.So i believe the boot seal may be the problem.
I can place a rag on top of the rear panel,close the boot and then remove the rag so this confirms that the boot seal isnt a tight seal.
Is this case for the seal to keep the gas out or just the water and does anybody else have thiss issue.

Thanks Paul