FM Transmitter - no sound, please help?

[Frimblypoo]

Please can someone help

Bought FM Transmitter from Maplins to use with ipod nano. Set it up, frequencies matched, deffo tuned in as static went but there was no sound, sounded just like stereo with mute on - clean noise if you no what I mean.

Tried different channels, different levels of output noise from ipod and stereo. Nothing. Same with generic mp3 player

Took it back. Bought another off eBay. Exactly same problem.

Tried both devices on house FM stereo, still same problem.

Would be grateful if anyone could suggest anything? (Google can't!)
Thank you

Am I right in thinking these plug into the 3.5mm headphone socket?

Yup
It's a mystery. Both the expensive one and the cheap one same problem. Just thought I must be missing a really obvious trick.