My sensor speaker started croaking/not working so I decided to replace it. Having looked at various advice on the net, the common theme seemed to be removing the back seat and all sorts of panels etc which is not a small job. Anyway, none of that is necessary. The speaker is located in about the middle of the rear parcel shelf and can be accessed by lying face up in the boot(trunk). On the underside of the parcel shelf, you will see x2 5 inch diameter holes in about the middle. Unless you have extra speakers fitted, which I dont, they will have a rubber type sound proof sheet in them. They are only a little bigger than the holes and are only held in by a slightly sticky glue and can easily be lifted up. The speaker is located between these holes so you just reach in and there you have it. There are 2 nylon pegs securing the speaker which easily push up to release it. You can see the 2 small holes that are kind of diagonal where the speaker pegs fit.
It's a simple 5 min job and if your sensors appear to not work, having checked they make a very slight clicking sound when putting your ear to them, it's worth doing before relacing the box of tricks in the spare wheel well which will set you back £150 from Jaguar.