General Motors Corp. plans a new version of its OnStar driver-support service that it hopes will give carbuyers another reason to put a GM car or truck on their shopping lists.

OnStar will begin offering a new feature called Turn-by-Turn Navigation, which uses automated voice prompts to guide drivers to their destinations and builds on other safety and communication services already packaged with OnStar.

The service is the first of its kind in the market and appears to be an answer to complaints about in-dash mapping systems that are sometimes distracting and confusing to operate.

"We take the simple part of our brand very seriously," OnStar President Chet Huber said in an interview. "We don't have any interest in being a company that requires you to take five steps to program one of our services."

The Turn-by-Turn feature is part of a seventh-generation overhaul of OnStar, which debuted at the Chicago Auto Show in 1996 and will return there this week to celebrate its tenth anniversary.

It will be introduced this spring on the 2006 Buick Lucerne, Cadillac DTS and Cadillac STS and be phased in on other GM vehicle models over the next two years.

GM is looking to OnStar to set its vehicles apart in a crowded market. While luxury auto brands Mercedes-Benz and Lexus have similar services, GM has pledged to make OnStar available on every vehicle by the end of 2007.

GM is also adding features to OnStar to ensure it remains relevant to consumers who are becoming more tech-savvy. Late last year, for instance, OnStar began offering a vehicle diagnostic service that runs remote check-ups on a subscriber's car or truck each month and e-mails the customer a report with its findings.

Other standard features such as remote door unlock, stolen vehicle assistance and notification of airbag deployment to medical institutions have helped OnStar gain 4 million subscribers since its launch.

Despite its successes, OnStar must be careful to nurture its growth carefully as it builds on its core lineup of services, said Jim Sanfilippo, industry analyst with AMCI Inc. in Detroit.

"The challenge is that the technology is starting to go really fast," he said. "And like anything, they will need to explain it."

To that end, OnStar began running new ads last month pitching its new vehicle diagnostic system, and it is planning a marketing launch for the Turn-by-Turn system in March.

OnStar, part of collaboration between GM, EDS and Hughes Electronics Corp., started as in-vehicle communication services company with around-the-clock operator assistance. In 1999, it became a wholly-owned subsidiary of GM.