Car owners in the UK are leaving possessions worth £1.6 billion in their vehicles, often on display, making them easy targets for potential thieves.
Clothes, sunglasses and sat nav systems are the most common valuables that are left at risk inside cars, the research indicates.
More than half of drivers (51%) leave belongings in their cars overnight on a regular basis, and many fail to hide them from vision in the car's compartments.
Motorists should check theircar insurance policies, to see what kind of cover - if any - is offered regarding theft from their vehicle.
The research by Populus found that 77% of motorists leave sunglasses in their car, and 28% often leave clothes and shoes.
Other items left for thieves include sat navs (20%), mobile phones (6%), iPods and MP3 players (5%) and cash amounting to more than £20 (6%).
The people studied who left valuables in their vehicles during the night thought their items were worth about £85 on average. But according to the British Crime Survey data, the typical cost of items stolen from cars is closer to £200.
Data from the British Crime Survey revealed that in over one in three (34%) theft from vehicles cases, thieves accessed the car via an unlocked door.
Copyright Press Association 2012