OK, i'll try to give you some instructions, since i did replace the heater core and the heater blower motor on '86 Audi 5000 two weeks ago. Let me tell you this - it's not easy and prepare to work on it for about 4 hours if you have all materials and TOOLS ready. I followed instructions provided from other people who've done the job before but it wasn't as easy as it sounded on paper. And some instructions didn't even match what i saw in the car.
*** Inside: Remove the dashboard(tach, speedo), AC control unit, Stereo, glove box...everything that can be removed. and then try to pull out the whole dash. You may still do the job without removing the whole dash but you'll have less room to operate and disconect the heater ducts.
*** NOTE: If you ONLY want to bypass the heater core and not replace it at this moment - Ignore the work "Inside" the car - work only under the hood. ***
Go outside and remove the hood. Yes, remove it. Let me warn you - when you install the hood back you need to align it, that's why, before removing it, mark with black marker the exact location of all bolts. After you remove the hood, remove both wiper blade arms and then remove the black "Audi" labeled cover. Remove the wiper blade motor and mechanism attached to it.(4 bolts total) Disconect all visible cables in the area. *** With a powerful, strong vacuum suck all the junk collected in the area. You'll really find lots of it. Under all the junk, on the bottom, you'll discover two drain holes that most likely are cloged with leaves and dirt. They can be seen behind your sttering rack too. ***
At this point, You should be able to see the two hoses attached to the heater core....they are located between the black fuse box and the actual heater assembly. (heater assembly => white plastic, rounded thing glued to the wall, going inside the car) *** If you want to only bypass the heater core and not replace it now...this is where you diconnect the hoses(from heater core) and connect them one to another using a "U" connector or another invention. ***
If you want to replace the heater core and the blower motor, or only one of them, you should pull out the whole heater unit out. (the whole white plastic thing that looks glued to the firewall) The heater core is inside it and you need to split the heater assembly in two parts to get the heater core out. I know, it sounds complicated but this is how we did it on my friend's Audi. Some people here may know an easier way but honestly, i couldn't see it when i was working on the car.
I'm not sure if you want to replece the heater core on your car and i'll stop here for now. If you need more help or more tips how to the job, just let us know.
Good luck!