My stereo is not producing full sound in my 1990 780. I can faintly hear some sound but it’s quite faint. Fusses appear to be good behind stereo and amp under dash. I imagine that there are fuses to check under the rear amp as well. I just did not see them. Looking for help. Thanks.

Does your 1990 Volvo 780 have the factory installed stereo or an aftermarket stereo?

Did the stereo fail all at once or was this a gradual or intermittent stereo.

If factory original install from 1990, you will need to somehow test the:
- Head unit in the dash (What make and model radio is in the dash)
- Separate amplifier (If factory original by now the Mitsubishi and Alpine auto audio gear Volvo installed can fail. Factory amps are always powered.)
- Speakers?

Do you have the factory installed Dynaudio speakers?

I can't (yet) find the 1990 780 Wiring Diagram. I'm looking at the Mitchel Service wiring diagram for 1990 940.

See here (copy and paste URL):
http://www.v8volvo.se/mekartips/volvo/Filer%20940/_Filer_940.html

On this web page download or open the PDF file "Volvo 940 1990 Michell.pdf".

The PDF contains three different pages with different stereo configurations.

We'd need to know the factory radio model number.

Door and rear hat or parcel shelf speakers are generally easy to replace by removing the grill over the speaker. Yet Volvo used speaker from Dynaudio and these speaker are nearly offset and not square (or round) screwed in mounting points as do aftermarket speakers you'd by today.

Anyhoo, there you go. Awaiting you reply. Please reply!
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Jonathan Harshman Winters III: The Mightiest, Greatest, & Most Powerful North American Comedian & Comedic Actor in Perpetuity!

Failing all else, you might examine and test all your speakers.

I cannot vouch for your car, but after I removed door panels a couple of times, I could do it in a short time.

Once, I cleaned all the window switches in our '88 745 GLE in less than 3 hours, working at a leisurely pace.

The speakers are connected with a complex circuit to match impedance. Perhaps a wiring diagram might help you find a short or open.

A short across speaker terminals , or in the speaker wires could do that.

Agreed - I would start with the speaker terminals then test the speaker wire for a short and/or break in the insulation.
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Will I buy another Volvo??? We'll see....