I wanna install an aftermarket subwoofer and amp iin my A4 but I'm not able to find any wires in the trunk. I tought it should have them already in there in case u wanna hook up an aftermarket kit. I have the power outlet in the trunk already but I don't know where to get the remote signal from, and the audio signal from my stereo. any ideeas, help?

 

Its not that easy. Yousee car manufactures don't include such a nice convenience as aftermarket hook up options because there is no way for them to know the exact needs each such system may have.
Your best bet (if you have never done this kind of thing before) is to take it to a professional and have them do it for you. I know you may not want to pay for what you and others may be able to do. The up side to having a pro do it is if they screw it up, they fix it.

Having said that if you still want to go with doing it your self, its not all that hard.

Start off by deciding on whats your over all goal will be. IE sub/s size, power needs to run the sub/s and if stiffening caps will be needed.
Once you do that you can go to a retailer and by amp hook-up kits that should have most of what you need.
If you plan on keeping the factory stock head unit, your best bet is to get a line level to low level adapter so you can tap factory speaker lines at the rear speakers to make your low level sub signal (note: you will have no sub level control if you do this, but its about the only way to tap into a factory system to get the signal you need).
once you have that you will need to run power from the battery of the car to where you want to have the amp. It is important to note that all cables are not equal here. You will need a cable big enough to supply the amount of power you need. If you think that you can just tap into the factory wire harness and grab power from any place you are WRONG! it will put a huge strain on the amp and factory wiring.
When you run your new guaged power wire make sure to fuse it at near the battery of the car, I also like to fuse it near the amp as well.
Now you will need to find ground. No I don't mean the ground you stand on,I mean electrical ground for the car. Find a bolt near the amp, something hidden that is metal to metal contact, best if it is on the frame of the car as near to the amp as it can be. Use some thing like sandpaper or a wire brush to knock off the paint so you have the best connection for the ground. Once again cable size is important, maybe no so much as the power but make sure you have decent cable for this. (ground is where most car stereo systems pick up noise from the vehicle) If you by a amp wiring kit it will have what you need.
The last wire to run will most likely be the hardest to do. You need to run a remote trigger to turn on the amp. In the olden days this was a simple as finding the factory power antenna lead and tapping into that. on todays cars its a little more complex, as not all cars have a external power antenna. I have included the best information I could find on the factory harness for you it says you will need the acc lead. check below for specs on it.
Afterthat is done, you are done!

here is the best example of a wire diagram I could find for you. It is off of a 2005 a4. (note I highlighted the connections you will need)


Audio/Mobile Video
WIRECOLORPOLARITYLOCATION
BatteryRED/WHITE(+)RADIO HARNESS
AccesoryBLACK/BLUE(+)RADIO HARNESS
GroundBROWN(-)RADIO HARNESS
IlluminationGRAY/BLUE(+)RADIO HARNESS
LF Speaker (+)BLUE/WHITE
LF Speaker (-)BROWN/WHITE
RF Speaker (+)RED
RF Speaker (-)BROWN/RED
LR Speaker (+)BROWN/WHITE
LR Speaker (-)BLUE/WHITE
RR Speaker (+)BROWN/RED
RR Speaker (-)RED
Steering Cntrls
THIS VEHICLE MAY BE EQUIPPED WITH OPTIONAL ONSTARĀ® TELEMATICS EQUIPMENT. USE AN APPROPRIATE INTEGRATION HARNESS OR INSTALLATION PROCEDURE TO ENSURE PROPER FUNCTION OF THIS EQUIPMENT.
Radio SecurityYES
Security
WIRECOLORPOLARITYLOCATION
BatteryRED(+)IGNITION SWITCH HARNESS
Ignition 1BLACK(+)IGNITION SWITCH HARNESS
Starter 1RED/BLACK(+)IGNITION SWITCH HARNESS
All Door TriggerBROWN/WHITE(-)IN HARNESS IN DRIVERS KICKPANEL
Trunk TriggerWHITE/BLACK(-)IN HARNESS IN DRIVERS KICKPANEL
Parking LightsGRAY/BLACK(+)AT VEHICLE LIGHT SWITCH
Convenience
WIRECOLORPOLARITYLOCATION
Headlights-LoWHITE/BLACK(+)AT VEHICLE LIGHT SWITCH
Remote Start
WIRECOLORPOLARITYLOCATION
BatteryRED(+)IGNITION SWITCH HARNESS
Ignition 1BLACK(+)IGNITION SWITCH HARNESS
Ignition 2RED(+)IGNITION SWITCH HARNESS
Accessory 1BLACK/YELLOW(+)IGNITION SWITCH HARNESS
Starter 1RED/BLACK(+)IGNITION SWITCH HARNESS
Anti-Theft TypeUNIVERSAL IMMOBILIZER
Anti-Theft DescriptTHE KEY SENDS AN RF SIGNAL TO THE BCM MODULE THROUGH AN ANTENNA LOCATED AROUND THE IGNITION CYLINDER
TachometerGRAY(AC)AT IGNITION COIL
Speed SenseWHITE/BLUE(AC)ABS MODULE
ABS CONTROL MODULE LOCATED ON DRIVERS SIDE FIREWALL. PIN 33
Parking LightsGRAY/BLACK(+)AT VEHICLE LIGHT SWITCH
Brake LightsRED/BLACK(+)AT SWITCH ABOVE BRAKE PEDAL
Modules
WIRELOCATION
Please select another category
Doorlocks/Windows
WIRECOLORPOLARITYLOCATION
Power UnlockRED/BROWN(-)IN HARNESS IN DRIVERS KICKPANEL
PowerLockGRAY/WHITE(-)IN HARNESS IN DRIVERS KICKPANEL


I hope this log winded explanation helps.
JM

 

umm i don't mean to be a jerk on the forum, but your gonna need a direct tap into your battery, you can't just use the 12v cigarette adapter to run your amp off of especially if your going to run a system thats going to be throwing more then 500watts into an application. the remote turn-on wire is the wire that tells your amp when to turn off and when to turn on, usually you wire it to something that turns off when you stop running your car and goes on when your car does and always stays on,

i haven't opened up my dash yet but i'm gonna assume that there isn't a nice convienent remote turn on wire like in aftermarket set ups, however after looking up the accessory blue wire should do just fine

i'm skipping alot of stuff but if you get the point where you have the power wires run to the trunk just always make sure your speaker wires and power wires stay as far apart as they possibly can or prepare for some very annoying static as you listen to your music

 

As I said, you need to run the power cable to thebattery and fuse it at both ends.
As to hum or static, its caused (most times) due to poor ground. Audi's are notorious for ground loop problems because of a floating ground connection.....