[Barfly]

@JDleaf
Advice from an ex tv and audio engineer State Service Manager Pioneer Electronics WA in the early 1980's...
Listen closely with your ear to the speaker and play an audio clip with speech in it or tune to a radio station with speech. If the speaker has "poled" it will sound distorted. "Poled" means the voice coil is rubbing on the central cylinder of the magnet assembly causing the distortion.
Another possibility is the right front channel amplifier has developed a fault. The two faults should be easily distinguished by an experienced audio engineer, but not perhaps by a layman.
It is extremely unlikely to be a wiring fault.
The first thing to do would be to get hold of another speaker of the correct impedence and substitute. Most car sound speakers have 40ohms impedence, but Nissan could have put anything in there.... You could swap the left and right door speakers to ascertain, but that entails stripping down both doors-a bit of a pain. My advice if you are not particularly handy yourself would be to take it to a local car sound specialist for diagnosis.
If you don't mind getting down and dirty you can save yourself a pile of money by swapping out the left and right ones.

Good luck.

 

[Barfly]

@JDleaf Garry is correct, it would not be economic to get such a speaker re-coned, although there are specialist speaker people who can do the job. I would suggest looking on-line Amazon/ebay/aliexpress and get a new one, secondhand or crashed vehicle ones from a scrap yard may have other issues, the doors and windows on the Leaf are not exactly waterproof!!!
If you hold the speaker close to your good ear and gently move the cone in and out you will probably hear a scraping sound. You should not be able to hear that!

Just look on the back of the speaker for it's impedance, this is critical. It should be a standard 4 Ohms, but knowing Nissan it could be anything, I have seen some weird and wonderful values in some cars even 120ohms on an Aussie Ford derivative.
If there is nothing on the speaker, don't bother trying to measure the dc resistance, speaker impedance is an ac measurement at a nominated frequency, usually measured around the 1000hz frequency. You will have to dig into the parts manual, I have a link for them in my dropbox account here: https://www.dropbox.com/sh/kdrmel9bo4vpox7/AAAzZFJnW_iO_EBoa9cAc96Oa?dl=0

You will also have to ensure the replacement is physically able to fit the cut-out in the door panel. There are a zillion different sizes.