I have 1994 Cadillac Eldorado Northstar can the back fire wall freeze plugs be changed with engine in the car? I changed the front one. There hard to get to and no room.

Hi Steve: You do like to have fun don't you.
If you have access to a hoist it may be a lot easier to try this.
My first question is, "How do you know that they need to be replaced"?
If it froze a bit and the plugs have popped, it may be a lot easier to finish removing them. If you do get them out, getting the replacements in will be tricky since as you know, there's not much room. So if you have the old ones out and are ready to put the new ones in, do a "dry run" to see how it will go. Is there room to slide a socket into the new plug and be able to tap it into place? If not, is there room and something available to pry against to push them in?
Last resort, with both the new plugs and the holes clean (like really clean) put the new plugs in your freezer overnight. You need them cold to do this. Wipe the receiving edge of the hole with sealant. Then, quickly get the COLD plug and tuck it into place, tapping gently in around the edges if the's not room for the socket trick.
If you're really masochistic, you can try all of this working on the ground under the car on ramps or jack stands. If you do, WORK SAFE! Block the wheels, transmission in park, and hide the keys.

It is also possible that you could try most of this working from the top. There would be a few parts to remove to give access. Bottom line, whatever works PROVIDED NOBODY GETS HURT.