Never could get an answer from TOMTOM about either my mount that was supposely to be shipped sometime ago or the hum. Found on another website the solution and it was so very simple. Install a ground loop isolator between the audio output of the 910 windshield mount and the aux input of the stereo. Stopped the noise instantly. Here is a link for one example of an isolator and the one I purchased.
http://www.radioshack.com/product/in...ductId=2062214
These are also available from a lot of electronic stores or audio shops. I am removing the rca plugs and replacing the four with two mini stereo plugs that will make a nicer install. The unit from Radio Shack only comes with one "Y" adapter and most will need two. The two mini stereo plugs cost the same as the adapter. You will need some soldering skills and a little finese as the wires are rather tiny.