First, what is your car LHD or RHD (you do not have e.g. Country info in your profile)? Why I ask, I suspect that depends on steering wheel placement side, PDC buzzers may land on different sides of the car.
I was extracting rear buzzer from saloon version RHD (when I war retrofitting front PDC to my SW - buzzers are the same), and rear buzzer there was located on right side under the panel say beside the rear right door glass, on its lower part.
In my SW I never needed to touch/see rear buzzer, but I suspect it is in the same place as in saloon, see picture below top left corner I would bet (there is cable going under, which may be from PDC). But as I said, this is RHD car.
And, why you think it is buzzer? your mileage clock is displayed all the time or blinking ?(asking in case PDC unit died/is disconnected/fuse blown etc)
Do you have front PDC, and so activated distance display for from obstacles between the display betwen clocks? just asking to check if the PDC unit itself works OK, so if you do, does it behave right showing distances to obstacles?
any errors displayed re PDC? have you got chance connecting/diagnosing it through CAN with PC?