Pull the speaker out and inspect it. If it was brand new, was it broken in properly? It's not good to turn up new speakers real loud until they are broken in with a few hours of play time on them at normal/not loud volumes.
If the cone or surround is ripped... usually it has to be replaced.
Is that speaker in the existing rear deck? Are you running a sub in the trunk?Do you have that speaker on an amplifier?Are you using a stock HU with an LOC?