Please help diagnose problem with stock infinity

[petero]

I have a new-to-me 1999 MS Limited with the stock infinity sound system.  The audio system appears to have never been modified, and mostly works well, but there's a problem with the right front speakers.  So, there is a 6" in the door, 3.5" in the dash, and tweeter in the door.  The 6" speaker is barely audible.  It's getting some signal, but produces very little sound.  When I plug a good speaker into the wires for the 6" door speaker, it does the same thing, so this is not a problem with the speaker.  The 3.5" males plenty of sound.  It has the same volume as all the other speakers in the truck, but it appears to have been blown out.  There's a lot of distortion, and it was unplugged when I bought the car, because of the poor sound quality.  The tweeter doesn't seem to make any sound at all.

So how is it that the audio signal is getting to the 3.5" speaker just fine, but seems to be severely attenuated by the time it gets to the 6" speaker.  Any ideas for what I should check?

Thanks in advance for any help you can provide!

--Peter

[petero]

I did a little more diagnosis, but still haven't figured it out.

1. The problem is not with the speaker, since any known-good speaker does the same thing when plugged into the door.

2. I did a couple tests of the wiring. First, I connected a speaker using the + wire from the 3.5" dash speaker and the - wire from the faulty 6" connector, and the speaker worked fine. Then I connected the - wire from the dash and the + wire from the door, and again the speaker worked fine. Both wires in the door seem to work with a partner from the 3.5" speaker connector, but they just don't work together. Weird, huh? I also measured 17 ohms across the wires from the door to the wires from the dash, so they are neither shorting nor broken. So, to this amatuer, it doesn't seem to be a wiring problem.

3. Seems like the head unit must be OK, since all four channels (front & back, left & right) have at least one good working speaker (remember, the 3.5" speaker on the drivers side front works). The head unit just provides a pre-out to the amp for each of the four channels, right?

4. So what about the amp? Could the amp be providing good signal to the 3.5" speaker, but bad signal to the 6" door speaker on the same channel? If so, what can I do about it? Can I replace the amp? How do I find an amp that will work with my stock infinity system? What would you do?

Thanks for reading. Please help!