1989 240DL rust forming help me diagnose!
I recently was given my first car, a 1989 240dl. A dream come true. I quickly learned that it had a giant rust hole beneath the driver's seatish area(see pics). I am almost done patching it up but here is the thing: I don't know how it began. Here's what I know: it is only on the driver's side, and after spraying the doors and windshield with a hose for 5 or so minutes I determined that there is no problem with the seals. The previous owners say that at some point it was sitting for a couple months with a water bottle spilled where the rust hole is but would that really be enough to make this hole? Also, there are little rust spots forming all around the sheet metal (including a few holes in some places[this is also pictured from below]). This makes me think it wasn't just a rock chip or some other random damage to the sealant. Anyway, I am almost done with the patch but I don't want the rust to reform in a year or something! I don't know much about fixing cars (because I have never before had one) so hopefully someone who is more knowledge can tell me what the heck happened.