there should be two screws underneath the head unit that hold the plastic trim over the head unit. remove them then pop the whole plastic piece out of the dash, your gonna have to unclip the wire harness's that go to the 4wd switch, the rear wipers, defrosters, etc. After that i forget cause i havnt had my stock HU in for a long time. but it should be pretty obvious, im guessing a couple screws here and there.
All this is assuming that a '98 will have the same dash as a '97. im not really sure.
hope this helps,
later