Parking sensor system problem.

[bigles]

Can anyone help me locate the key parts of the basic parking sensor system please? It sounds like my speaker is on its way out or the cabling may be. The beeps when reversing started to crackle and now seems to be intermittent. The reversing lights come on when reverse is selected. I am an electrical engineer so can replace the speaker myself if I could find it. Can anyone help?

Les
2007 A5 3.0TDi Quattro