How do i replace rear speakers for a 199...

If the rear seat does not fold down, it needs to come out prior to removing the rear deck lid cover over your speakers.

You can remove the rear seat by wrestling with the seat. You will have to work with it to get it out. Don't force to hard, but give it plenty of muscle and attempt not to break the clips that hold it in place.

Generally an "up" and "out" motion should do it.