For a long time, I did not like that cars to my side could not tell if I was signalling. A couple of years ago I took a pair of parts bin side marker lights, the little orange ones about 1 x 2 inches, and mounted them in the ends of the bumper and used some bits and pieces of spare harnesses to splice them into the turn signal circuit. I had to modify the back of the lenses a bit, to make them fit a simple round hole. Then I used a bit of hot melt adhesive (industrial grade stuff I use on my speaker work) to make sure they would stay put..
Now I have installed the 85-87 corner marker lights with my round headlights and a relay to make them flash along with the signals. This solution is applicable to all the type 85 cars from those years. I have wired the relays so they are switched by the difference between the parking light circuit and the turn signal circuit, which is easy because they are both grounds when "off."
It is also necessary to load the flasher unit with a light bulb filament in order for it to flash at the correct rate, so I used a couple of parts bin (ex-87 5000?) sockets that would fit the original bumper lens and house dual filament bulbs. This way the bumper light is also a park and signal device. Without doing this you would have to wire them as signals an give up their parking light function, which is not too bad since you will still have the corner markers anyway.
You might ask, why not just substitute a dual filament socket in the corner lens? I tried to find one and nothing in my parts bin came close enough. The socket is different than the bumper one, in that it has a different depth of protrusion into the lens itself.
So to make this work you drive the corner bulbs as shown in this diagram, and make sure there is still a filament loading the front turn signal wire, one way or another.
Here is the circuit diagram for changing your Audi coupe/4000 (and many others) corner marker lamps to dual-duty signal and marker lights: