Amazon will begin delivering packages to Volvo and GM cars, even when drivers aren't around.

The service will be free for Amazon Prime members with an active GM OnStar or Volvo On Call account and is available in 37 U.S. cities and surrounding areas. The companies have plans to expand the service to more locations.

The move adds another location where customers can receive packages, which is key for Amazon as its retail business depends so heavily on deliveries. The e-commerce giant has experimented with several types of partnerships, such as recent expansions into delivering groceries and meals.

And Amazon has grown, competitors such as Walmart, Costco and BJ's Wholesale have developed their own delivery programs.

Users will need to download the Amazon Key App. The service will only work on 2015 model year or newer Chevrolet, Buick, GMC and Cadillac cars, or a 2015 model year or newer Volvo equipped with the Volvo on Car service. That includes more than 7 million GM cars, Vijay Iyer, director of communications for GM Global Connected Customer Experience and Urban Mobility, told CNBC.

The service does require access to your car, which some customers may obviously be reluctant to grant. Amazon uses an authentication process to ensure the right delivery driver is at the right location with the correct package, unlocks the car, and locks it again once the delivery is complete.

Last year, a security firm identified a flaw in the Amazon's similar Amazon Key home delivery system that would let a driver temporarily disable the security camera and roam a customer's house. Amazon responded by saying it screened its drivers and would send a patch to fix the flaw.