Hey im trying to hook up my subs but have never installed to the rear speakers through a convertor before. Just want to keep my stock head unit. Any how to's or anything like this? Also anyone know a good place to run the positive wire through the firewall or somehow get around it? Thanks.

 

Although this is possible I would not recommend connecting an amplifier (im hoping your subs are amplified and aren't being driven by the head unit!)in this way, the sound quality will not be as good as RCA input (AKA Line Input) and there is also a risk of your head unit overheating as essentially it is dropping the voltage of the speaker output to line level, providing more resistance to the head units internal amplifier resulting in it having to work harder.
If you still wish to keep your stock head unit and take this risk then your best option would be to look for an amplifier which has speaker (high) level input. This is a much safer option as the amplifier will attenuate this input preventing your head unit from overheating.

As for the power cable, you should find a ready drilled hole with a rubber grommet and existing wiring in the firewall/bulkhead in the passenger foot-well provided kindly by Audi which easily fits large gauge cable through, all you need to do then is remove the plastic kick plate carpet covers you will find running down the side of the seats and tuck your cabling underneath, lift the back seats and feed into the boot (or trunk as I believe you yanks call it).

Hope this helps. DeeBee

 

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 https://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 so you don't need a high to low level line converter. This is taken from the tech article which I have added 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.

As for running power wire, inside the rain tray I ran my 4 gauge wire through the top circle in the picture. I was still able to cut a hole through the grommet and use it to seal out moisture. I used a coat hanger to fish it down into the foot well then fished it down the drive shaft tunnel under the center console and carpet, under the rear seat and then into the trunk. I love coat hanger. My favorite tool. I ran ground to one of the bolts underneath the rear seat.

If you have any questions fire away.

Doug