Faulty Speaker Wiring: Inspect the wiring between the radio and the speakers. Ensure that the wiring is intact and there are no loose connections or damaged wires. If any damaged wiring is found, repair or replace it accordingly.
Speaker Output Settings: Confirm if the speaker output settings in the replaced radio are configured correctly. Some radios have options to enable/disable certain speakers or adjust the balance and fader settings. Make sure that the speakers you're having issues with are not accidentally set to mute or the balance setting is not skewed excessively in their favor.
Amplifier and Speaker Impedance: If your vehicle has an existing amplifier connected to the speakers, there is a chance that the speakers themselves are still fine, and the problem could lie with the amplifier or impedance matching. Verify that the speakers and amplifier are impedance-matched to ensure proper functioning. If mismatched, there may be compatibility issues leading to sound irregularities.