I normally put it on the small alloy/carbon/wood trim on the left side of the wheel, or all the way in the right corner just at the bottom of the a-pillar. However I have done 1 mounted from the courtesy light unit on the roof & another hidden in the ashtray. It's all personal choice.
Best way to check is with a multimeter if you have one to make sure it's the right place. It's the same place as I used on my 57 plate jetta when I first fitted my new radio. But since them I've upgraded again to a full canbus adapter - so no need anymore to have the fuse there.