Thought I'd add a bit on AHC sensor maintenance, so here goes:
It is fairly easy to maintain the AHC sensor, particularly the rear one as it is easy to remove. When you have the sensor in your hand, with the lid off, you can see how simple it is. It consists of a body with a shaft through it, and a 3-pin connector at the back of the body. At the outer end of the shaft there is an arm which transfers the suspension movement. The inside end of the shaft has a little arm or rotor with two brass sliders/springs which provide the connection to the carbon resistance traces, which are on the inside of the lid of the sensor.
Then the important part: The connection between the 3-pin connector at the back, and the carbon traces on the lid, is provided by three little brass coil springs, which are just squeezed between the connector at the back, and the lid at the front. These springs corrode. Pull them out of their little holes and clean both the bottom of the holes (connects to the 3-pin connector) and the springs. You might need some contact cleaner and some emery paper.
The problem with the sensor is that the lid, with it's O-ring, doesn't seal well. And it looks like the lid bends slightly. So you have to seal it better. I haven't found the right method yet, tried with vaseline and with silicone. It would possibly be good to put an aluminium plate over the lid, to support it so that it doesn't bend, and then use longer screws.
Pictures:
This is the sensor on it's bracket, without the lid, in a vice. The part between the jaws has the hole where the cable clip is attached. The part pointing straight up is where the bracket is fastened to the car body with two small bolts. At the bottom of the photo you see the arm which is connected to the leading arm of the rear suspension via a ball-joint.
Pic #2 shows the "rotor" with it's two sliding springs, and the three holes for the springs. at the left side, towards the connector, next to the three holes, you see a bit of "pollution" in the o-ring groove, where moisture has made it's way in. It looks like the pressure of the three spings is to much for the lid, and makes it bend. You can also see traces of corrosion at the bottom of two of the holes.
This pic shows the lid with the carbon-traces, and the O-ring. And the three springs, two of them a bit corroded at one end. In this pic I have filled the sensor with contact cleaner.
To locate the rear sensor, you just look in above the front part of the rear left wheel, when the car is lifted to High. Or crawl under the car (unless you have access to a lift or a pit). I found it easiest to crawl under in front of the left rear wheel. Removing the wheel could make it easier, although I never did. It's easy to see which part it is that senses the suspension travel. Undo the linkage (one little nut, 10mm spanner); the electric plug and detach the cable from the bracket; and remove the two bolts holding the bracket to the body. Voila, the sensor is yours.
(I removed my AHC 
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