[MagicRat]

I have a almost-identical van. The inside panels unscrew quite easily. Just take your time, start with removing the visible screws and remove the panels. Sometimes some screws may be hidden.
It's an easy job. Just keep track of how the panels came apart so you can put it back together again.
But do not be surprised if your original speakers are not 6x9's. My rear speakers are not that size.
Depending on the interior layout, the factory speaker cavities may be a different size. If yes, the 6x9's can be made to fit, but a pro should do that. Or new original-size speakers are likely available.