I spent this afternoon changing all my door speakers. If you had posted your question earlier I could have taken photos while I was doing it
It took me a little while to find all the fixings but the facelift Omega must have one of the easiest door cards to remove (undamaged) of any car I have owned
1. the little plastic grille in front of the door handle (and hides the tweeter) - pop it off. I used my fingers but you might need to use a screwdriver.
2. finger plate behind the doorhandle (includes the tweeter mounting). There are two plastic prongs on the back of it (behind the tweeter) that clip into the door card. I used a screwdriver to ease the top one out and then the bottom one unclipped fairly easily. Pull the door handle half open and the fingerplate should come away from the doorcard. Once the fingerplate is off you can undo the Philips screw that is hidden behind it.
3. armrest fixing screw. Open the storage bin in the armrest and unclip the lozenge shaped plastic cover (it's the bit that your fingernails scratch when you press the button to open the storage bin). Unscrew the Philips screw hidden behind it.
4. Philips screws. There are 7 or 8 screws around the edge of the door card. All are easy to see (two of them might be behind plastic caps at the front of the door if the caps haven't already fallen out)
The top of the door card clips onto the bottom edge of the window opening. The first doorcard I removed completely but I didn't bother with the other 3, I just held the card up enough to get to the speaker.
WARNING
I think the front doors have impact sensors inside the doors to trigger the air bags. If you only touch the speakers and speaker wiring it won't affect the impact sensors. If you discover chaffed wires at the door hinges then be careful not to disconnect any wires that you cannot identify.
If you do disconnect any air bag wiring and switch the ignition on then you will need to get the airbag warning lamp reset afterwards as it won't go out on its own.