Here is a picture of one rear frame rail. This is after the trunk itself was removed.
Where they connect to the leaf spring in the rear is a pretty common place for them to rot out.
You will have to look up from the bottom of this since everything is still on car.
The frame rails will be on both sides of the gas tank and run up to just under the rear seats.
You don't want any rot in these frame rails.
Unfortunately, though, many have some rust in those rails.
Here is one from the engine bay. With the engine out. Less common for rust on these but not unheard of by any means.
This picture is the interior. You can see where the front drivers frame rail connects to the transmission crossmember.
This area will get rust where the two connect. You can just make out the connection point on the bottom left side.
I will see if I can find some more pictures of these areas for you.
I have noticed you have asked several times where to look.