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Metra 40-GM21 Antenna Adapter

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

16 Reviews

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Item # 12040GM21

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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.

Plug and play it works

Doug, Knightdale, NC

What's in the Box:

  • 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).

Metra 40-GM21 Antenna Adapter Reviews

Average Customer Review(s):

(16 Reviews)

Plug and play

Doug from Knightdale, NC on 7/7/2017

Crutchfield Customer Review (What's this?)

Plug and play it works

Pros: Easy install made rest of installation easier


Metra 40-GM21 Antenna Adapter

Jeremy L. from Swansboro, NC on 2/21/2017

2006 Hummer H3
Crutchfield Customer Review (What's this?)

Exact fit. High quality and fast shipping. I have been buying from Crutchfield since I was a teenager and I will continue to buy from them.



Metra 40-GM21 Antenna Adapter

Richard from Tavernier, FL on 9/25/2016

2003 Chevrolet Blazer
Crutchfield Customer Review (What's this?)

Did not fit..... I ORDERED THE WRONG PART. They refunded the part immediately, no questions asked

Pros: Great customer support and prompt service

Cons: None

Metra 40-GM21 Antenna Adapter

jg from North Carolina on 9/7/2015

Crutchfield Customer Review (What's this?)

cheaply made had to electrical tape together




Anonymous from Indiana on 1/20/2015

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

jroman from rockford,il on 9/7/2014

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

joe from fredericksburg on 2/22/2014

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

AlcoholicProgram from Pittsburgh, PA on 9/1/2013

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

Timi from San Mateo, CA on 8/17/2013

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

Hap from MT on 7/4/2013

Crutchfield Customer Review (What's this?)

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



Too long

Ken from Saint Paul, MN on 5/19/2013

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

C P from Tuscaloosa, Al on 5/6/2013

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

John W. from Cuba, NY on 3/26/2013

Crutchfield Customer Review (What's this?)

Easily attached to my FM modulator.



Worked great

TSub from Central KY on 1/29/2013

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

Billy M from Sumrall, Mississippi on 1/2/2013

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

Michael from Flagstaff AZ on 2/21/2009

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

Product Research


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

Our Product Research Team

At Crutchfield, you'll get detailed, accurate information that's hard to find elsewhere. That's because we have our own in-house Product Research team. They verify what’s in the box, check the owner's manual, and record dimensions, features and specs. We stay on top of new products and technologies to help people make informed choices.

Q & A

Customer Q&A

Crutchfield asked  
Why did you buy this?

always has had good service and quality material from crutchfield. Why shop anywhere else? [ Doyle W.  Jun 21, 2017 ]
Fits my truck [ phillip  Aug 14, 2016 ]
My goal is to add an AUX port to the factory installed stereo system. This item is one of two items needed to complete this goal in my 2005 Pontiac G6 GT. [ Paul  Jul 29, 2016 ]
I choose one but both showed up on the order. If I don't need both delete whichever one is not needed. Thanks John [ Tech Brown  Jun 12, 2016 ]
price [ ronald  Jan 12, 2016 ]

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