I’ve got the Sonichi one too. Yes you need an extra aeriel. Mine’s clamped to the inside of my van high up where it works fine 99% of the time. The tiny co-ax plug into unit isn’t very secure, when it occasionally falls out the signal reduces by 50 to 70% I reckon.
It doesn’t plug into the Aux, you tune one of your radio presets to it, which works very well. And can I also recommend,
Radio 4
XFM
Some of the Absolute stations.
for those times when 6 Music is a bit shit. Mainly when Mary Anne Hobbs is on.
edit – Just remembered, the most annoying thing about mine is the ugly and bulky suction mount cradle that comes with it. I’d like to try and find a different way to mount it and that would put me off buying another.