Stereo head unit replacement?

What is the unit you installed? What is feeding the signal to the OE amp, SPDIF. From the description it could be a bad amp, but I'll look at the head unit and do you have a link to the actual adapter box you used, I don't see a maestro listed on idatalink.

To narrow it down to the amp or something in the interface, the easiest way would be to go to the factory amp and use a speaker to test the outputs directly. I doubt that all the speakers failed, possible but doubtful. So, take the speaker you are going to use and connect it between like pairs. So, to test the right front you would connect to FR+ and FR- for the left rear it would be RL+ and RL- I would look at the back of the connector, you might be able to just slip the wires into the back of it to test. If say you get sound at the amp but not at its installed speaker then you would need to get to that speaker, but I sort of doubt that is the issue. The SPDIF unit doesn't really show how it gets the audio from the head unit, does it plug in to the audio output jacks as individual connections? The install book doesn't show it.

Okay, it sounds like they are sending the "speaker out" into the converter which turns it into the SPDIF signal to go to the amp. Check the RCA outputs. If those all work, then I suspect that box has an issue. Perhaps run it through the set-up again. The only other option would be to find something with the DPDIF signal and feed that to the amp and see if it works then.

It's a shame the factory unit is not working; it would be a lot easier to test if you had a working unit to plug in. If it all worked, then you would know if it was a bad new part or something in the old amp.

That is the fun parts of modern electronics. The catch 22. Being a 15 you might try going to a yard and seeing if they have a used one you could try. The local you-pull-it here gets $25 for the average radio. At least that would let you test. I don't know of an easier way to test than that really.

It's the unexpected stuff that throws a wrench into things. At least you don't have the fun of many newer vehicles that have the sound system so integrated that none of it can actually be replaced with anything that isn't stock. There are vehicles out there that use the "radio" for everything.

Think about a car with a system like the Tesla. They use a single tablet screen to control everything. Then because it's all on a single network, one item has a failure, and it shuts it all down. Door handle fails, car won't run, back-up camera fails, you need a tow. Screen malfunctions, you lose heat, radio, heated seats, navigation, all other controls are gone as well. Your car isn't quite that bad. My process for "used" parts is to get electronics, which don't control the vehicle, from wrecks, very few cars are taken off the road because the stereo died. For control parts I get those off wrecks, they were running when wrecked after all. Same with engines and transmissions. If they have no physical damage, they should be operational. I also cheat, I have a battery box that I take with me into the yard and I'll hook it up and then bypass the power to test things like a window or seat motor or to check something like the radio I made up input connectors that I can plug into the power feeds for the item and test it.