660 Suction mount (Power/FM Receiver Cable) wiring

Your 660 most likely came with a Garmin GTM20 cable that was long and had three LED lights.
They made a newer GTM20 cable that was shorter and had no lights. I think that is a better made cable and less apt to break, I've owned both and like the newer one better.

Power cable connections can fail if not secured firmly that will relieve stress on the mount, whether a power mount or connecting directly into the Nuvi.

To avoid this I wrap the cable around the arm of the mount (I use bean bag), so there is no stress on the connection.

Long GTM20 Traffic Cable (With 3 LED Lights)
http://www.amazon.com/Garmin-GTM-20-Power-With-Subscription/...

Shorter GTM20 Cable (No LED Lights)
http://www.amazon.com/Garmin-GTM-20-Lifetime-Traffic/dp/B001...

If you can live without the traffic feature, should you have future problems you can buy this far less expensive cable that will work on the 660.
http://www.amazon.com/Garmin-010-10747-03-12-Volt-Adapter-Ca...

The basic rule for cable shielding is "Ground the cable shield at one end. This eliminates the potential for noise inducing ground loops".

You will probably find that the cable shield is connected to equipment (the Garmin receiver) ground . Connect an ohm meter to the shield you "folded back" and the exposed metal on the USB plug on the other end and you should read continuity.

Know the issue. Ground loops are caused when two devices are connected together to "earth" or ground through a single conductor. Current flows through the shield when one has a higher potential than the other and this is what causes the problem. The "bazooka" grounding method of attaching a shield at one end works best when the shield is connected at the end closest to the grounding point. Both ends of the shield can be connected if the devices themselves are attached to a single grounding point. Ground loops are really troublesome in electronics installations where there is either a lot of radio frequency energy generated or power is used.

I had in mind the way the cable of one of my nuvis connects to it's cradle but after I looked at a manual for your nuvi 660 I see the cable plug at the mount-cradle end is way different.

Given the 660 configuration and re-reading what you did, I believe your cable shield is grounded at the cigar plug end.

I should have said that you should see connectivity from the folded back shield and the ground side of the cigar plug.

It still would be ok (and expected) that the shield is not connected to any thing in the cable plug at the mount-cradle end of the cable.

It's still working, and the ground is not same as shield of connector.....

If it simply was a ground shield for radio receiver in the power plug, I could buy that... except the ground shield extended about 1/2" past the plastic covering and molded connection point for the connector....
That was what sent me off looking for pulled off solder connections on the metal ground pieces in the plug for suction mount...

Finding no solder marks on the metal pieces or anywhere else would lead to the idea that it's a shield, but then why the ground braid being so far past the plastic cover and mount assembly....

UNLESS it's the plastic covering on the wiring that shrunk! It did seem that the wires were much longer (More pulled out) of the mount.. By about the same 1/2" that the ground braid is out past the plastic cover...

My revised guess is it is the plastic covering that shrunk....