Just did this retrofit, works great.
In order to access the PDC/Buzzer location you need to remove the front trim around the steering wheel.
Remove the black "AIRBAG" plastic cover that's on the left of the steering wheel by prying it off using a plastic trim remover.
Remove this part using plastic trim remover, only tabs that's holding it it.
2. On a LHD car, driver side, remove 3 screws for front trim.
T10 torx I think.
One bolt here:
One here:
And then you should be able to pull it off towards you while sitting in the driver seat.
Make sure to disconnect the wiring for the headlight switch and the interior light without breaking the wires or connectors. They are held in pretty firmly so slight force might be required but everything after the point of removing the one bolt on the side and the two underneath, should be by pulling.
PDC and buzzer are fitted here, behind the fuse box:
PDC module will be quite obvious where it is when you look underneath since there should be a bracket if you have rear parking sensors (4k) since before. Buzzer was quite tricky to be found but there is a moulding for the buzzer that you can feel with your fingers on the backside of the fuse box.
Here's the PDC bracket holder that you simply pull out the red part and then pull down on the bracket and the PDC module should be free to be pulled down as well:
Here's my old 4k module that I removed:
You should also see the OEM cable pass through in the firewall by locating this harness almost where the PDC module is located.
Wedge the red part of the foam away and you should see the pass through with a rubber gasket.
In order to access this pass through from the engine bay you need to remove the battery to expose the firewall and also the rubber grommet with the harness that you saw from the inside:
Audi were kind enough to incorporate a cable pass through (blue part) underneath the battery tray that you can ues to route the fron PDC harness so you can feed it through the firewall and connect it to your 8k module inside the cabin.
I used the nipple-looking part of the rubber grommet to pass through the new wiring so I didn't need to mess with the OEM harness.
Easiest way to feed the cable was to press the nipple so it appears on the inside, make a small incision and then start feeding some kind of wire that you locate in the engine bay, tape the front PDC harness to and start feeding into the cabin. Once the PDC wire has been feeded to the cabin, make sure to pull back on the wire from the engine bay so that the nipple goes back into it's original place and form. Depending on how small incision you made, you can tape off the rubber nipple and passthrough with some tape in order to be 100% sure that it's sealed so no water can get into the cabin.
Here's how it looked like after I was able to feed the wire inside the cabin.
After that it's just a matter of connecting the 6 wires to the T18 connector that is used for the front PDC and that goes into the 8k PDC module and connect the front buzzer (2 wires) to the already existing T26 conncetor that was previously connected to the old 4k module.
Connect the T26 and T18 connectors to the 8k module and put it into it's designated spot.
Before reassemblying everything, code the new 8k module and connect the parking sensors to see if they work.
I did my long coding using OBDEleven but people usually do the coding using VCDS.
After that the only thing that's left to do is to reassemble everything.
Btw. If you already have the 4k rear PDC like I did in my PFL 8V S3, replacing the button panel should be enough since it's already pre-wired to work with the new button panel for 8k PDC. Same thing with the CANBus High/Low so only modification you need to do is to add two wires to the existing T26 connector and assemble the new T18 connector that both go into the new 8k PDC Module.