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Metra 40-GM21 Antenna Adapter

Works with select 1987-up General Motors vehicles — factory male to female Motorola antenna connector

Item #: 12040GM21

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13 Reviews | Ask Questions

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Metra 40-GM21 adapter

The Metra 40-GM21 adapter lets you connect an aftermarket antenna cable to the factory radio. It has a female Motorola plug on one end and a right angled male GM antenna plug on the other. This adapter can also be used when installing a wired FM modulator with the factory radio. Please enter your vehicle information or call an Advisor at 1-888-955-6000 to find out if this adapter is compatible with your vehicle. Warranty: 60 days.

Note: Most vehicles will require additional installation parts — please identify your vehicle if you haven’t done so. You can also call an Advisor for more information.

Metra 40-GM21 Antenna Adapter Reviews

Average Customer Review:

13 Reviews

Metra 40-GM21 Antenna Adapter

Written by: jg from North Carolina on September 7, 2015

Crutchfield Customer Review (What's this?)

cheaply made had to electrical tape together




Written by: Anonymous from Indiana on January 20, 2015

1998 Saturn SC1
Crutchfield Customer Review (What's this?)

Did not fit. The connections were backwards.


Cons: Did not fit, connections were the opposite of what my GM car required.

perfect fit

Written by: jroman from rockford,il on September 7, 2014

Crutchfield Customer Review (What's this?)

perfect fit adds satelite to factory radio using xm module

Pros: keeps factory radio adds satelite with module

Cons: none

worked like a charm

Written by: joe from fredericksburg on February 22, 2014

2000 GMC Yukon XL
Crutchfield Customer Review (What's this?)

my only gripe with gm vehicles is their ant. cables.does anybody know the reason for using small cables?



Simple and easy

Written by: AlcoholicProgram from Pittsburgh, PA on September 1, 2013

1997 Cadillac DeVille
Crutchfield Customer Review (What's this?)

Possibly the easiest thing to install so far. The GM Antenna adapter by Metra could not be any better. It does exactly as intended. If the low price scared you, or might even have you jump right into purchasing it, think again. The GM Adapter by Metra is the best product for its use and makes installing a new system that much easier and better.

Pros: Works perfect, simple install, very inexpensive.

Cons: Absolutely none.

Metra 40-GM21 Antenna Adapter

Written by: Timi from San Mateo, CA on August 17, 2013

1998 Oldsmobile Cutlass
Crutchfield Customer Review (What's this?)

Good product but couldn't use it because I did not specify which one I needed.



Just the ticket too

Written by: Hap from MT on July 4, 2013

Crutchfield Customer Review (What's this?)

This fits the radio and lets a regular antenna work on GM products.



Too long

Written by: Ken from Saint Paul, MN on May 19, 2013

2001 Chevrolet Tahoe
Crutchfield Customer Review (What's this?)

Worked like a charm

Pros: Plugged in tight.

Cons: Too long. Had issues with stuffing it an all the other wires back. behind stereo.

Great Adapter

Written by: C P from Tuscaloosa, Al on May 6, 2013

Crutchfield Customer Review (What's this?)

Great adapter, had no problems installing in my 2002 Chevy S-10.

Pros: Great adapter, easy installation

Cons: None

Metra 40-GM21 Antenna Adapter

Written by: John W. from Cuba, NY on March 26, 2013

Crutchfield Customer Review (What's this?)

Easily attached to my FM modulator.



Worked great

Written by: TSub from Central KY on January 29, 2013

This worked great for taking my Suburban back to a stock radio. The adapter made it easy to put a factory style radio back in since the antennas are somehow different. Had no issues with loose connections either.



Don't Know Yet

Written by: Billy M from Sumrall, Mississippi on January 2, 2013

2006 Pontiac Grand Prix
Crutchfield Customer Review (What's this?)

Had to use this with an adaptor for FM modulator of iPod. Did not fit very tight on a GM radio. Still works, but it seems like if ever hit a large bump in the road it's probably going to come out. But I may be wrong.

Pros: Works, so far.

Cons: Fits loosely in GM radio.

Metra 40-GM21 Antenna Adapter

Written by: Michael from Flagstaff AZ on February 21, 2009

2004 Chevrolet Malibu (Classic)
Crutchfield Customer Review (What's this?)

Why does GM use a weird miniturized Motorola antenna connector? Dunno, but they do. This is the answer (or if you are installing an XM modulator in-line, it is half the answer.)



Hands-on research

What's in the box?

Download owner's manual

  • 6.438" antenna adapter cable (one end terminating in a female Motorola plug and the other end terminating in a right angled male GM antenna plug).

Product Research


Compatibility: 1987 and later GM factory radio with mini connector to universal antenna adapter cable (factory radio to universal antenna)

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