When my ST3 arrived i took out the rear seats, rear quarter panels, no the mesh grills do not come off separately, and took the front door cards off. What i did was buy the large roll of Dyno matt...
Where the rear speakers sit i covered the outer car body behind the speaker in dyno matt and all over... Stuck the fibre padding (supplied) to the lower body, then i sealed the rear part of the car with hollow fibre wadding, so the large space where the speaker is was sealed off to the rest of the space towards the back of the car to act as an enclosure. I put a foam strip round the rear speakers and refitted it all. I then fitted dyno matt to the flimsy metal panel that the rear speaker bolts to, and in the surrounding areas, leaving enough for the front speakers.
Then i removed the front door cards, sealed any holes from the inner black cover, and again fitted dyno matt behind the speaker position and over most of the door body. Since the speaker sits on the black plastic cover this is adequate for insulation to the speaker.
Put it all back on and.... woooo! Increased bass, no tin sound where the bass hz is lost through the metal!! Since bass frequency is between 80-120hz it rattles the panels which absorb the sound which is what distortion is when it upsets the speaker. I can now hear the rear speakers far better, high range is crisp, but its the mid range and lower range that is vastly improved i think.
My car now sounds A LOT better, ST 2 & 3 and Titanium's are fitted with the premium speakers with the Sony stereo pack, but the body is the same as any other fiesta. When i met up with a fellow ST owner, i tapped on the rear quarter panels of my car to their car. Theirs was hollow, tinny sounding, mine was dense, thick. So not allowing the sound to escape.
Yes, get better speakers! And do not use the OEM Wires but then that means after market head unit, and then you start losing integrated features from the car and slipping down that route of complete ICE, so i left mine alone! Improve the insulation, its far cheaper for the benefits you get.
Hope that helps..