i have an aftermarket head unit, couldnt tell you

just get an aftermarket HU... better sound and looks much better

i have a 99 a4, with stock symphony unit and an amp/subs combo....what i did is buy the hi-lo adaptor(before anyone says anything bad about hi-lo adaptors, you haven't tried this one out!)....its called LOCB.2 and works specifically with Bose Systems to give you the Best sound with your RCA's....it will run you about $80, but the sound is amazing...! This is the route I took, but dont specifically recommend it to you since its expensive, just putting it out there for you as an option! Good Luck!

i spliced into the remote on the factory amp in the trunk to go to my aftermarket amp,spliced into my left and right rear speakers and ran them to the amp and then connected them into the sub. use an volt meter. turn power on the car,disconnect the stock amp and use ur volt meter to find out the 2 live ones, the remote should turn off when the radio power is off, and the other one will still have power, easier to findin out which is which,splice into ur remote from the factory and connect it to the sub amp, connect the stock amp back up,run ur power and ground like u do normaly do on any car audio and ur set. thats what i use and no complaints yet. sounds perfect. i would take pics to show what i did but its pising rain rite now.

There are tons of postings on hooking up an amp and sub to a stock Bose h/u - do a search. Here's a pic of my installation - feel free to PM with any specific questions.

Hi/Lo adapter is what you need. converts speaker wire into rca's, check pac-audio they make them, so does periphrial and some of the other stereo acc. companies. got 2 cvrs running off 1250 watts rms, just connected right into the hi/lo. with an aftermarket head unit the output will be far greater, it cuts down on the signal output when converting it. but yes it is very possible.

btw dont grab the signal behind the radio, you'll have to locate the factory amplifier and get the connection from there (non-bose stereos in audi's still have factory amps).

why shouldnt he grab the signal behind the headunit?

factory amp, i guess you could grab the signal from behind the headunit, but if you get any wire diagrams they'll probably read colors from the amp, so youd have to do the 9v speaker poping test to figure out which to tap into.

i already posted the diagram