Tweeters in the factory locations, just remove the screws located at either end of the dash (you'll see them with the doors open) and then there are clips along the length of the windscreen. Like a lot of commodore interior parts, it lifts out "unclips".
As for cross overs, mounting them in the doors is probably the easiest. Make sure all connections have proper ends, soldered, heat shrunk etc. Use some metal brackets, tape, glue, sound deadening, cable ties. whatever means you need to make them very secure. Mount them inside the door trim on the outside skin of the door.
As for an amp, you could look at something from out site sponsor, Joe from Shopping Secure. He has an awesome amp available which would run your gear very nicely. It would give your splits 70wrms @4ohm, and 250wrms to your sub @4ohm, enough to get it moving a bit, though a little more power in the sub department would not be bad. Here's the link
Shopping Secure - Ground Zero Iridium GZIA 4110HPX - (Powered by CubeCart) Its only $240.
Here is the Jaycar Response amp
Response Precision 4 x 100WRMS Class AB Amplifier - Jaycar Electronics
It is rated at 4x100 wrms, so your splits would take the front 2 channels with 100wrms each and the sub would get a very healthy 380wrms for your sub. It is only $250 as well, very cheap for the power and its of reasonable quality also. So all up you could get Amp from Jaycar, splits online, where are you getting your sub from? And that just leaves wiring.
All up going to be a very nice system mate.
