1999 Audi A4 1.8T w/Concert headunit
The real reason for this article is to help people out with the wiring diagram for the harness in the 1999 Audi A4's. That is the primary piece of info that would have helped me out online. Otherwise, the info here is pretty standard and this is NOT a super hi-fi set-up experience. This is a "I want the harness wiring diagrams to fix/replace my rear amp" experience. Hopefully this article makes you feel more comfortable about attempting this.
I purchased a pre-owned A4 sigh-unseen for a great price that came delivered with a missing rear-amp (and turbo clamp and ac belt, I quickly discovered! :) ). Although the dealer offered to get me one as soon as another A4 came through his lot, I opted to just install a third party amp rather than wait (after waiting 4 weeks first) and to avoid the fact that if the dealer pulled it from another A4, then someone else would have to deal with it. That just didn't seem right to me. And since the dealer was apparently dumping this otherwise good Audi for such a good price for the reasons explained above, I couldn't complain about this very simple and inexpensive fixes. However, the rear-amp fix, if I used an Audi part (8d5035401), which came as both speaker and amp only, would cost about $200 with no dealer mark-up. Since it was such a dinky amp (the base Concert speaker set-up), I figured a 200w 2-channel amp from anywhere would actually be better. I got one for $50 from Walmart, no less, to fit the bill.
The rear amp is a smallish thing, normally attached to the rear left speaker (sedan) in the rear deck plate. It appeared to me the amp had somehow been broken off the speaker as parts of the molding on the lower portion of the speaker were broken. The harness that comes from the head unit was just dangling. I searched for wiring diagrams for the harness but no luck, so I got out my trusty multi-meter, a small pc-speaker with stripped wires, and went to work. Here is the wiring diagram:
The brackets represent the slots that slid into the male harness end. The side with the outward piece is the piece I chose as the "top".
1: White- Switch
2: Red-Blue lead to Right speaker
3: Brown-Red lead Right speaker
4: Red-Blue (thicker): 12v
5: Blue-Green Right pre-amp from head unit. I am assuming they are pre-amps as I got a voltage reading of about 1v with the volume full blast. It was just enough to just barely run the speakers or the pc-speaker at full blast as well.
6: Red-Green Left pre-amp from head unit
7: Brown (thicker) Ground
8: 2 Brown-black Grounds for pre-amp leads.
You should probably check this out on your harness if you are replacing your stock amp (and/or speakers) to make sure. You know the disclaimer routine.
As you can see, there are no left speaker leads. My understanding is the amp/speaker setup comes with the amp having a direct connection to the left speaker terminals.
First I cut off the harness, stripped wires, and loose-wired all the wires to my new amp before mounting. Once I was satisfied again that everything was what it appeared, I proceeded to mount the amp with four galvanized zip screws to the rear deck from below. I made sure to attach it to double walled areas with nothing above it so they wouldn't come through the deck. I also removed the rear deck cover to facilitate in running wires, but it really didn't help that much anyway. I mounted the amp close to the left speaker as I had already mounted a 6-CD changer to the rear deck near the right speaker. I was able to put the amp quite a ways back and out of the way. Because the amp was so cheap (but still well overmatched for the crap base speakers Audi put in) and light that I am sure the 4 zips would hold it there fine forever. If you are doing this with a real amp that is heavier, I might suggest actually bolting the amp to the rear deck. Then I proceeded to really wire. Since my amp had rca jacks in, I had to cut up an RCA cable, strip wire that to the pre-amp leads. All the pre-amps and leads I finger wired together, folded over, and wrapped with electrician's tape, then wrapped again with a layer of gaffer's tape. The power and the switch I used orange (small) wing-nuts and then wrapped with electrician's tape. I did not use the #7 Ground/Common and grounded the amp directly to the rear deck. I don't think it matters to not use the harness ground, and perhaps I should have tested that... If anyone has a good reason you should, please let me know as I would be happy to know that if it helped anything out.
For amp-to-right speaker connection, I actually pulled into the wire run that came into the harness where it passed by the amp and ran it directly the amp rather than piggy-tail it from the harness point. For the left speaker, since I had the rear deck cover off, I measured my needed speaker wire length, cut it, then took out the speaker and soldered the wire length directly to the terminals. Then I reinstalled the speaker and ran the wire to the amp. This only required about an 10 inch length of speaker wire.
I tested everything, turned the gain almost all the way down on the amp (this seemed to match the front speakers best). Left it on flat (full range) for crossover (that seemed to be best as well), and left the bass-boost (loudness) on. I put everything back together and tested again. Great! The stock sound system in the A4 pretty much sucks, and this new amp pretty much didn't change a thing to that status I am sure (I have test driven other A4's with same sound system and they sound pretty much the same, sucky). But at least the rear speakers were working again. It made me really appreciate my "Sun & Sound" package Infiniti sound system in my 2003 Mitsubishi Eclipse! Wow, what a difference. But I would say that the A4 sounded about the same as my 2000 Honda Accord EX with the JBL speakers and stock head unit (again, nothing to rave about, but fine for just plain old drive along music/news).
One thing I noted: The right speaker was marked 4ohm. My amp was 8ohm or 4ohm bridged. Since my amp was cheap, but surely had much more wattage (even if we assume the real wattage of this 200w amp is only 40-60w/channel sustained that had to be more than the miniscule little amp that is installed on the stock system), hooking up small 4ohm speakers to an 8ohm amp meant that the speakers would draw more than it 'should', but so what. I think it will handle it.