The speakers should read near 4 ohms, +/- 1 ohm. A reading of zero means that there is a problem in the circuit: either a wire is disconnected somewhere (fixable) or the voice coil is toast (not fixable)... or your multimeter isn't working correctly and there is some other problem.
Try this: get a 9 volt square battery, about a foot and a half of wire, and some electrical tape. Cut the wire into two equal lengths and strip a half inch of the shielding off each end of the wires. Wrap the unshielded end of each wire around the positive and negative terminals of the battery. Now wrap the battery with the electrical tape so that the wires are held firmly in place around the battery terminals. You should now have a speaker tester. Hold one of the wire ends to one of the speaker terminals (doesn't matter which one) and then tap the other wire onto the other terminal. The speaker should make a sound like a "pop" when you touch the second wire to the second terminal. If it doesn't make a sound, the problem lies within the speaker... again, it'll either be a broken circuit or a toasted voice coil. If it does "pop", then the speakers are fine and the problems lies in the cars audio system or in the installation of the speaker.
Hope this makes sense and I hope it helps! (Disclaimer: I'm not responsible for anything, unless this helps you, in which case I am completely responsible

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