If the trim piece is the same as our 2004 Vectra (and I believe that it is), then there is a metal piece of trim that has screws in it holding the panels in place. There is a long plastic cover or trim piece that sort of snaps over the metal panel. It's tedious work, but if you proceed very carefully, the plastic cover piece can be removed.
Start by pressing hard in the very center of the trim piece while working a very thin screwdriver or similar underneath. The plastic trim does come off if you remove one side and then tug gently on the opposite.
Once the plastic cover is removed you can easily see how the metal trim is attached with screws. A simple matter to remove the screws then you can access the backside of the headliner.
Installation (putting the plastic back on) is a breeze.
There are several threads that discuss this here on a sister board...Winnieowners.com...here is one post...
Journey headliner - Winnebago Owners Online Community
Winnieowners is headed up by the same people as irv2...so not being disloyal :-) There were so many Winnebago posts that a second site was created.