I have an 1989 240 DL. Could anybody tell me what is the big metal "box" for want of a better description, is on the left side of the car just past the left rear tire? I see some rust damage on it and I want to know if I should be concerned. I tried looking for an undercarriage diagram online, but couldn't find it. Thanks for all the help.

 

Sounds like you mean the "butt cheek" ? If so, it's the spare tire well and the tend to get water trapped inside and rust starts. Nothing really structural to a point but should be dealt with [open drain hole and repair leak ~ usually a trunk seal].

 

It is between the left front tire and the left rear tire, closer to the rear. Could it still be spare tire well?

 

So, it must have moved? LOL!

I don't know, maybe the rocker panel? This would be the area below the side doors from wheel well to wheel well. If so, this would be an area that is structural in nature and should be looked at.