You are seldom if ever going to get a decent signal with the antenna setup available to vehicles. I'm surprised just how good it actually is, considering I don't have a Log Periodic antenna on a 50 foot mast pointing at the Tx

You have a pair of small, omni-directional low or no-gain antennae, at best at shoulder height (as on my X3 where they're in the window glass) or at worst at knee hight where mounted in the rear bumper (a-la E46) so unless you have good line of sight to a nearby transmitter will give you either no signal or perhaps just the high-power PSB multiplexes. As for using when driving, forget it. I tried it whilst driving past the Mendip transmitter, and it would still break up.

All this being said, I do still use mine quite a bit, whilst parked up having some lunch whilst out and about, and it does work OK for me.