so im trying to install a sub in my 03 a4, question number one is how to you take the stock head unit out? i know there are special keys to pull it out with but is that the only way too?
next question is, does the stick head unit have RCA outs? or an i going to have to buy a new head unit?

 

You need the keys to remove the "head unit". You can buy them for around $15 shipped, just look around.

 

I actually did this a couple months ago... Yes you need the keys and no there are no preamp outputs on the stock head unit. You can buy this converter box thing that you wire into your stock wires and it creates 2 rca outputs. I actually ordered this amp that came with it just cuz I was doing a sub only...

 

yea there are little recatangle holes 4 to be exact on the radio. Use the keys and pull out. The rca can be connected through any speaker. I connected my through my rear door speaker the wires are on the side column between the rear and front seat. I think the wires are purple and brown but not 100% sure.

 

If you have a Symphony system, look in your trunk on the right behind the side panel. You might be able to spot it through the vent. Its an amp. An amp that uses pre-amp (low level) inputs and a 5v turn on lead. Look around here. There are wire maps for this amp, and shows you where to get your low-level woofer + and -, and your 5v turn on.

If you don't have Symphony, dunno what to tell you, pretty sure all the amplifying takes place in the head unit on everything else. Personally I LOVE the integration of the stock radio, and its one of the best, flattest sounding head units anywhere. Its a shame that they had to put Bose speakers in the rest of the car. When I get ambitious, I'll be putting half way decent speakers in and swapping that bose amp in the trunk to go with my Boston Acoustics subs.