03 pontiac montana

To put a din-sized unit in there you'll need the Metra 99-2001 dash kit.   This is what it would look like (except just pretend that the shaft-style supports are not there when looking at the pic).  A double din unit will fit fine without the kit, but it's going to be tough to mount it unless you can figure out a way to fabricate your own metal side brackets (or cut them off the factory radio).    There's two screws holding the left side bracket into the dash, and one screw holding the right side bracket into the dash.  I guess there may be a way to modify and reinforce the 99-2001 kit; however, there is also limited depth in this vehicle and depending on how deep of an aftermarket head unit is installed, you may have to cut/remove the rear support thingy.  That's gonna make it more difficult to properly support a double din unit if it is so deep that it requires you to perform this modification.

I think this vehicle uses the class II wiring system, in which case you should use an integration harness like the BestKits BHA2004T, and relocate the factory radio.  You'll know it has this type of wiring if once you remove the radio, the door chimes stop working.  The plug itself is I believe shaped like this one, but if u get that BestKits harness it will include all the necessary wiring and instructions.

Finally, I don't know if they have looked at this particular unit but it's a combo unit that fits in a single din opening and would work as long as you used that dash kit.