hmm, you can do it yourself... you can take the whole piece of carpet out if you lift out the 900 or so (kidding) fasteners from the rearside of the seatbacks...
I would imagine if your carpet is mildewy your cardboard backing is probably shot too, rip as much of it off as you can if this is the case. I think you would be OK to use a dilute bleach solution to get rid of the mildew (not sure if its 100% colorproof, test it out first if you care about losing a little color) but bleach gets rid of mildew pretty well. I'd say, soak it overnight in bleach and some laundry soap, rinse it off well with a hose, and hang it out to dry (i doubt if the dryer is the best idea)
if your cardboard backing is gone, you can do what I did, get a 40" by 40" piece of thin plywood, then cut it in two and put some door hinges in it so you can still get at the spare tire. Then you can just use a staple gun or whatever you feel like using (velcro works too) to attach the carpet to the wood.
I wouldn't know these things if I didn't spend an entire day fixing what ailed my trunk, lol