Thats not true at all.
I put my Alpine Type-S 5x7 6x8's in without removing anything but the grilles that fit on over the speakers.
First you get in the back seat of your car and pry up the speaker grille. Be careful, they've got little tabs on them that break fairly easily. I happened to break one on mine but no big deal, you can't tell the difference and the grille sits still perfectly. Next, get the correct alen key that fits the screws around the speaker and take out all 4 screws that hold the speaker in. Clip the wires going to the speaker from inside the trunk, and then splice in your new connectors. Finally take the new speaker, drop it down into the hole from inside the car on the back seat, replace the screws in their proper place, and plug your new connectors into your new speaker. Repeat for the other side.
The rear deck speakers are a cinch to replace. Now it's the front door speakers that are a *****, i'll tell you what.

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Black 2005 Mustang v6, 5 speed, IUP, EUP and Leather.
Razzi 1/4 window scoops, Mustang GT Sway Bars, Flowmaster American Thunder Single Exhaust, Clear Front Turn Signal Lenses, Demolet CAI, X-cal2 91 tune, Hurst Comp/Plus Shifter, FRPP T-Lock, Ford 3.73 gears