Good grief, I'm glad I started this thread, because between all of you, you've given me the answer.
Years ago, I purchased a set of cheapo stereo headphones which pick up a signal from a little black box at the bottom end of the FM frequency, they weren't that good and much, much better ones have become available in the meantime, so they're at the back of a cupboard.
In fairness they were marked not legal in the UK, but the hire cars are generally in the US, Ireland and various European mainland countries, I wonder whether I'll get arrested in any of them?