If the other speakers are working and you've confirmed a valid connection to the speaker in question (at the harness), then it is likely either the speaker is dead or the wiring between the harness and the speaker is interrupted somewhere. In which case the best place to use a multimeter would be at the speaker itself.
It is a simple task to remove the door panel on an Impreza. You will need to remove a couple screws on the door handle and then proceed to pull the door panel out of a series of clips along the outer edge (this is what you would do for an 02 WRX, I am sure the instructions would be similar for a base Impreza).
If you're up for removing the door panel; once removed, you would first notice the physical appearance of the speaker. If it's cracked or ripped, it's most likely blown. To be sure, you could do a resistance test with the multimeter. Set the multimeter to the resistance setting to measure the speakers impedance. Usually you would see a setting of 20Ω which is in the range of what we are looking for. Hook up the probes to the positive and negative terminals of the speaker and observe the reading. If it is zero, your speaker is dead. See a detailed tutorial here.
If you are getting a decent reading (which varies depending on your speaker's impedance), then you know your speaker is fine and the wiring must be faulty somewhere. To be sure, change the setting on the multimeter to volts in the range of 12V (usually the 20V setting). Turn your stereo on so there is power and connect the probes to the positive and negative wires at the back of the speakers. If you are not getting a reading of ~12V, then you have narrowed the problem down to faulty wiring. This could be fixed by re-running a new wire from the speaker to harness, which could be a PITA if you're not into it.