Commenters say nearly every E30 M3 corrodes under the plastic battery tray from battery acid and trapped water, and it is expensive to fix because several panels meet there.
Corrosion under the battery tray
BMW E30 M3 (1986-1991) · mechanical
Commenters say nearly every E30 M3 corrodes under the plastic battery tray from battery acid and trapped water, and it is expensive to fix because several panels meet there.