Hello & Welcome to Just Answer,
Sorry I didn't see your question earlier and to hear of any issue you are having with your device connections.
What you will actually need is the "cradle" for this series of devices (700 series), where the GTM 20 connects, and then the device "mounts" on the cradle.
I have a link to Amazon where that exact cradle & suction cup mount are listed.
Click Here --> http://amzn.to/1J8pU9y
Note: These cradle & mounts are becoming scarce, but you may be able to find only at a well stocked store - that carries a LOT of Garmin devices and accessories.
I hope this helps. Please let me know if you have any more questions.
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Thanks, ***** *****