Did a google search for "putting leaflets in car windscreens" and came up with this.
Placed on Car Windscreens
In the UK there are no regulations preventing placing leaflets on car windscreens. However, be careful to ensure that you don’t accidentally damage the car in some way or trespass on private property.
To avoid damaging cars it’s advised not to place the leaflet under windscreen wipers. In addition, many people find this extremely annoying, particularly if the rain has destroyed the leaflet or they only see it after starting their car. Getting someone irate before they see the leaflet is not the best way to generate a receptive customer.
Also bear in mind that councils have been known to contact companies that have distributed leaflets in this way, requesting that they clear up any leaflets that have blown or been disposed of on the road.
With the above in mind the size of the leaflet is often the key here. Print thick A6 or A7 leaflets, even business card size leaflets, and place them between the rubber seal and the window glass of car drivers’ doors.