This hole was there when I bought the car. I removed the trim piece but whoever drilled this hole drilled through it as well. They also cut a slit about 8" long through the insulation leading to the cavity behind the seatback. I can't believe it would be used for porting a speaker. I've removed the entire back seat making sure if it were used for something illegal there are no forgotten items. There is something under the seat (looks like steel wool fibers) that has caused some odd surface corrosion to the steel under the seat bottom.
Pic:
http://www.andrewlitho.com/E28/039.jpg
Any takers?
Thanks.
Any idea why a hole would be drilled behind back seat?
From the looks of it, the weird shape is just from them using a hole saw to cut the hole. Maybe it would have been a perfect circle if they had the proper tools to do it! 
That could've been used for anything. Running wires for an amp, although you probably would've found a few more holes for the mounting of the amp. Or, it could've been used for porting of a sub, as you suggested. Maybe they were bored / drunk / high / all three and decided to play Doc Brown with the trunk area!
At any rate, it shouldn't have caused any structural damage to the car, so you should be okay.
EDIT: By the way, I checked out the rest of the pics of your E28. Nice iS! It looks like your car suffered the same type of previous owner as mine did: cheap bastard that paid for a shitty respray. I've got little spots of surface rust showing up on mine, too! It appears that you have a very clean car, otherwise. Keep it maintained so that it'll be a good base for modification!
[Edit by Shifty on [TIME]1109567793[/TIME]]
That could've been used for anything. Running wires for an amp, although you probably would've found a few more holes for the mounting of the amp. Or, it could've been used for porting of a sub, as you suggested. Maybe they were bored / drunk / high / all three and decided to play Doc Brown with the trunk area!
At any rate, it shouldn't have caused any structural damage to the car, so you should be okay.
EDIT: By the way, I checked out the rest of the pics of your E28. Nice iS! It looks like your car suffered the same type of previous owner as mine did: cheap bastard that paid for a shitty respray. I've got little spots of surface rust showing up on mine, too! It appears that you have a very clean car, otherwise. Keep it maintained so that it'll be a good base for modification!
[Edit by Shifty on [TIME]1109567793[/TIME]]
Thanks, Shifty.
I can't imagine why they needed a whole that big, but its there.
I think all but the deck is original paint, but I'm not sure about the deck not being original. Carfax states it was involved in a rear end collision in the '90s and with the wierd paint bubbles on that edge I'm guessing it was resprayed. Car was registered in FL and AL until 2002 and I bought it in 2003...been in storgage until now. The rust on the decklid edge wasn't there until I stored it. I've just pulled it out for the first time this month. Just now starting to catch up on the regular maintenance items.
It will be at 5erFest this year.
[Edit by csp23 on [TIME]1109640345[/TIME]]
I can't imagine why they needed a whole that big, but its there.
I think all but the deck is original paint, but I'm not sure about the deck not being original. Carfax states it was involved in a rear end collision in the '90s and with the wierd paint bubbles on that edge I'm guessing it was resprayed. Car was registered in FL and AL until 2002 and I bought it in 2003...been in storgage until now. The rust on the decklid edge wasn't there until I stored it. I've just pulled it out for the first time this month. Just now starting to catch up on the regular maintenance items.
It will be at 5erFest this year.
[Edit by csp23 on [TIME]1109640345[/TIME]]