The pillar mounted aerial receives radio waves so that you can listen to the radio in your car.
The pillar mounted aerial is mounted on a pillar bracket, which fixes to the door pillar of your car, next to the windscreen.
The pillar mounted aerial is connected to your car stereo by an aerial cable which houses the conductor needed to receive radio reception, and the pillar mounted aerial is literally an extension of the center conductor inside the aerial cable.
The pillar mounted aerial will last longer with careful and regular cleaning, which helps to avoid damage from a buildup of dirt. The pillar mounted aerial will usually last around 4-6 years before you need to buy a new one, but are relatively easy to replace, sliding in and out of the door pillar. The pillar mounted aerial may, of course, get damaged and if it snaps, will naturally need replacing. If the pillar mounted aerial is still intact but you are experiencing poor reception and sound quality, getting worse over time, then this is a sign that you need to replace the pillar mounted aerial. It is possible that your car will still be able to receive FM radio when the roof aerial has stopped working but if your car is unable to pick up a signal on the AM frequency then it is very likely the aerial is broken.