I do a lot of driving and I'm noticing, especially with Audis, the number of cars that have had their windscreens changed; I can see them a mile off and tell immediately from the absence of a rain sensor gel pad - or coating - between the glass and the sensor itself.
There's a claim that the sensor will still work without it, but with the pre-formed silicone pads ranging from £8-23 each or the acrylic ones averaging at around a fiver each, naturally only the person expected to pay for them would claim they're superfluous. They're not, and the sensitivity of the unit is lessened if the gel coating is not there.
My aim is to raise awareness of this small detail and if anyone is having a windscreen replaced, irrespective of who does the job, make sure the gel pad is replaced. The three options are:
1. Silicone. The most expensive option, but OEM standard.
2. Acrylic. Cheapest, but unless the installer knows how to get around it, will always show bubbles.
3. Regeneration pack. Takes time but it is a 2k system which is formed in-situ; pretty much the pack to carry for multiple applications.
More about the subject here (blog).