The LF speaker (door) in my XC90 does not operate. Has anyone had this concern? What was the trouble? Wiring or speaker?

 

Make sure the speaker is plugged in. You may want to swap it with one that works. You may also want to post in the XC90 section.

 

Before you start, consider voice-matching. Voice-matching is key to effective home theater sound. If your speakers do not produce a similar tonal quality and share the same harmonic characteristics, the sound will be disjointed when it travels from speaker to speaker. With voice-matched speakers, the sound moves effortlessly around you, creating a seamless surround effect. The best way to ensure voice-matched speakers is to choose speakers from the same "family" of speakers, by the same brand. If you'd like to add center and surround speakers to an existing pair of stereo speakers, and you don't think that series is in production anymore, call Crutchfield or the manufacturer to see whether speakers from a newer series would be tonally similar.
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Had same problem... left front speaker did not work.

Bought another amp... still not working.

Found below link:

All Front Speakers And Left Rear Speaker Not Working - Volvo Forum : Volvo Forums

Basically, there is an in-line box for Sirius / Bluetooth install that fails. Just unplug the box... connector goes directly into the amp. It is located under the passenger seat (US). Three bolts hold the amp into the floorboard.

Remove the amp. The connector (green, about 16 wires) on the opposite end of the fiber optic connector (small, 2 wire) is a bypass connector... you will see similar green one if you trace it with your hand.

Unplug the connector from the amp, and unplug the "other" connector. Put the "other" one into the amp.

Done.

Disclaimer: one possible solution... many potential causes including bad amp, bad head unit, bad speakers.