Finally got around to fixng a problem with exhaust fume coming in to the cabin when doing some full-throttle driving. Here are a few reference pics, and notes if you intend on tackling this yourself. The trunk vents are about $12USD each.
On all e39 sedans there are 2 trunk vents, they are one-way and are designed to let air flow out from the cabin when the fans are on. When they get old the rubber flaps loose their seal and cause an undesirable smell from the exhaust to enter.
The new vents before some labour takes place :
The vents are held in by 6 plastic clips, and are near on impossible to remove from inside the trunk, or from underneath in behind the bumper. To install the passenger side vent, the Audio and NAV system needs to all come out as the supporting brackets are in the way. Off comes some trunk trim, the Audio and NAV system, and then finally the rear bumper. There are 3 press-in-type plastic fasteners holding the rear bumper to the rear wheel liner, they are a pain to remove unless you remove the rear wheels. 3 nuts on each chassis leg inside the trunk come off, unplug the PDC sensor, some careful pulling on the bumper and she's off
The old vents are removed by pressing in the bottom 3 clips from in between where the bumper sits and the lower rear quarter panel. E39's are getting on so these vents where well stuck even with the lower clips pressed in. A rubber mallet and some hitting did the trick.
The old vent about to get the bash :
Pic of the old vent removed, and Audio and Nav gear brackets removed, and the wirey mess left behind.
Installation of the rear bumper can be tedious with one person, but can be done. The 3 nuts attaching the bumper support to the car can be loosened to adjust how much gap you have between the bumper, and the lower tail panel. Recommend putting some masking tape around the rear quarter panel edges to protect the paint work.