TMC reception for the likes of TomTom,Garmin, Navman etc is not all that good, with either the "strings" or the aerials built into the power cable.

http://www.ge-tectronic.de offer various adapters that use the car aerial for TMC reception - I've used them on my Roomster/Navman set-up - much better reception and pick-up . Bit pricey though!

Myself and Mike Alder from PocketGPSWorld did some experimenting with this a couple of years ago on the TomTom TMC units - if you are handy with a soldering iron it is a fairly easy job, all you need is some coax, connectors and a capacitor to stop any DC voltage getting to the TMC.

Costs under a tenner, the connectors being the most expensive bits!

Makes a huge difference when connected to the car aerial!