E30 Battery box rust

[kworben]

Anyone experienced rust starting in battery box area ('89 325i/2dr)? Has rusted right through to the rear valance. Found this on a car that I am considering for purchase and is other wise in excellent rust free shape (low mileage/garaged/original non-enthusiast owner). Any ideas on the cause and how to make a proper repair? Also, need an exhaust system. Any recommendations?
Thanks,
Ken

[shogun]

Maybe the airation hose was not connected. Best is you weld a new piece of metal in.

[indymdm]

is the carpet of the trunk wet after a rian as well......and if so does it have a stock spoiler on the trunk if so might try washers on the bottom of the spoiler mine had same problem