Metra Axxess GMOS-01 02-Up Onstar Harness Adapter - Retains OnStar

In this review the Metra Axxess GMOS-01 is evaluated as a practical harness adapter for installers working on non-amplified GM vehicles from 2002 onward. The bottom line: it keeps critical factory functions like OnStar and dash chimes intact while providing adjustable output and accessory power, making it a straightforward choice when replacing a head unit in cars without factory amplifiers. Installers who need a pre-wired solution and USB update capability will appreciate the simplicity and focused feature set.

Key Features

• OnStar retention: Retains OnStar functionality with adjustable volume output so voice prompts remain usable after head unit replacement.
• Chime retention: Keeps the vehicle chime with its own adjustable volume output so warnings and reminders stay audible.
• Accessory power: Supplies a fused 12-volt 10-amp accessory feed to power aftermarket radios reliably.
• Pre-wired convenience: Includes a pre-wired ASWC-1 harness interface connection for easier integration, reducing installation time.
• Updateable interface: USB update interface compatibility allows firmware updates without reworking the install.
• Amplified vehicle guidance: Provides clear options for amplified systems via the GMOS-04 or an amp bypass procedure to avoid signal loss.

Who It's For

The GMOS-01 is aimed at mobile electronics installers and knowledgeable DIYers working on non-amplified GM vehicles who want to retain factory features after installing an aftermarket radio. It is especially useful where OnStar and audible chimes are required to remain functional and where a compact harness with accessory power is preferred.

Those with amplified factory systems should look elsewhere or plan to use the GMOS-04 or follow the Axxess Interface Amp Bypass Schematic, since this unit is intended for non-amplified setups and requires additional steps for amplified vehicles.

Pros & Cons

Pros

• Retains both OnStar and dash chime with adjustable volume, preserving factory convenience and safety alerts.
• Provides a fused 12-volt 10-amp accessory output that simplifies powering an aftermarket head unit.
• Pre-wired ASWC-1 harness compatibility and USB update interface make installation and future updates easier for professionals.
• Clear guidance for amplified vehicles gives installers options rather than leaving them to guesswork.

Cons

• Not a direct solution for amplified vehicles without the GMOS-04 or following an amp bypass procedure, which adds steps to some installs.

Specifications

Brand	Metra Axxess
Model	GMOS-01
Compatibility	Non-amplified GM vehicles, 2002 and up
Accessory Power	12-volt, 10-amp (fused)
Features	Retains OnStar, retains chime, retains RAP feature
Update Interface	USB update compatible
Included Wiring	Pre-wired ASWC-1 harness (ASWC-1 sold separately)

Our Verdict

The Metra Axxess GMOS-01 is a focused, practical adapter for anyone replacing radios in non-amplified GM vehicles and who needs to keep factory OnStar and chimes functional. It offers useful installer conveniences like USB updates, a fused accessory feed, and pre-wired harness compatibility, delivering solid value for typical aftermarket radio installs. Choose the GMOS-04 or the amp bypass schematic only if your vehicle uses a factory amplifier.

Frequently Asked Questions

Will this work with a GM vehicle that has a factory amplifier?
If the vehicle is amplified, use the GMOS-04 interface or bypass the amplifier per the Axxess Interface Amp Bypass Schematic included in the documents.

Does it keep OnStar and the dash chime working?
Yes, the GMOS-01 retains both OnStar and the vehicle chime and offers adjustable volume outputs for each.

Can the interface be updated in the field?
Yes, it has a USB update interface for firmware updates without removing the unit from the install.