You can tap into the rear connector to get low level output. Here is a DIY I made for doing it. You can use the sub amp's built in crossover to control cut off and the gain to control the loudness when you balance it with the rear speaker's loudness. Unfortunately you won't be able to control it with the deck with the current set up as the rear set of RCA's signal are being used for two sets of speakers.

I know this is hind site but in the future if you were planning on adding a sub and wanted to control it with the deck, buy one with 3 sets of RCAs. Then you won't run into this problem with not having full control and functionality.

"As far as adding a sub, I couldn't handle not having one after picking up my A4 at the end of 2009. After much research and looking through the tech article found here http://www.audiworld.com/tech/elec.html, there is an easy way to get line-out to your sub amp and it's right at the driver's side rear speaker. This is taken from the tech article which I have added to and taken out less important info.

First things first. This worked for me but do this at your own risk. I will not be held responsible if you screw something up. If you don't understand something, then ask before you start snipping wires. This installation does not require you to cut any of your car's wiring so please don't. Tapping means stripping a section of insulation off of the wiring leaving the bare copper wire showing at which point you can solder another wire to it and wrap your work with electrician's tape.

These wire colors worked for me but you may want to verify that they are the same before starting by looking at the back of your wiring harness off the stereo. My OEM stereo had a diagram stuck to it with the wiring diagram.

The rear amp is a smallish thing, normally attached to the rear left speaker in the rear deck plate. There is a connector plugged into the side of the speaker when you drop your drivers side rear seat down.

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) (2)
(3)
(4) (5)
(6)
(7) (8)
________[ ]________

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.
6: Red-Green Left pre-amp from head unit
7: Brown (thicker) Ground
8: 2 Brown-black Grounds for pre-amp leads.

In order to get line out, you just tap into the pre existing wiring using Scotch locks or stripping the wiring insulation and soldering the connection. These instructions will assume you are soldering the connections.

1. Take a set of RCA cables and cut one set of ends off making sure there is enough length to reach the amp.

2. Strip the end exposing about an inch of both the inner core and ground (outer metal sheathing). Do this for both ends then twist the grounds together from the left and right channel.

3. Strip the insulation off of the Blue/Green, Red/Green, and 1 of the Brown/Black wiring from the connector leading to the rear speaker using a razor knife. Do not cut the wires.

4. While you're under there, go ahead and strip the white wire on the connector for the amp's remote turn on.

5. Get a length of small gauge wire for your remote turn on and strip one end.

6. Get your connections ready for soldering by twisting the right RCA's inner core to the Blue/Green wire's stripped section. Then twist the left RCA's inner core to the Red/Green. Then the RCA's twisted ground from the left and right channel to Brown/Black. Finally twist the remote wire to the white wire.

7. Solder and tape up your work.

In order to get power to the amp, I ran a heavy gauge wire from the battery to the trunk. I ran ground to one of the bolts underneath the rear seat."

Doug

 

wow, thanks for the info, that's a project for down the road. thanks for all the advice and help cyber! you're awesome.

 

I'm glad Cyber got you squared away,this community is full of helpful and nice people,can't say the same for a lot of them but there are a few that are generous with their time and spend most of it helping people and not trolling and calling people names and being mean and doing everything else,ecept answer the posters question,lot of those here but i just ignore them and concentrate on the ones that don't mind helping and are nice.

 

so very true, i do wanna thank you and the people would assisted you in your install as well morpheous.

i used your videos to get me started!

 

Tonight i tried installing a avic-p2300dvd in my 2002 audi a4 w/bose. The factory cd player I have is a symphonyII. I am using the 70-1787 Metra Hardness and the antenna adapter. I have hooked up everything according to the metra guide and pioneer guide but i can not get my unit to power on. I started checking the wires and harness to make sure everything is connected correctly. I noticed that in some places the harness doesnt have a led where the factory harness does. Any ideas? I havent got one light to turn on the Pioneer head unit.

Thanks!

 

I have a 2003 b6 and I had to splice into a switched 12V wire to get the radio to turn on. The factory radio does not have a switched 12V wire so you have to find one and tap into it. I found one in the first plug of the HVAC control unit (left plug). Now it powers up just fine.

 

This thread helped very much, thank you!! I also thought I would share my experience. The scoche harness worked pretty well, but I ended up cutting the two RCA cables and attaching connectors to hookup the rear speakers. This worked, but the speakers would completely turn off after a few seconds. I pulled out the multimeter and discovered it was the car doing so, not the deck. After fiddling, I found that if I disconnected the ground for one rear speaker but left the positive connected, it did not turn off the speakers and they both still worked in the rear. Not sure why, but it's working!