Ford Mondeo parking sensor malfunction

Hi all,

I tow with a Ford Mondeo estate and ever since I've had it the parking sensors won't work properly. They beep when I put the car in reverse, even though there is nothing to beep at! It's been back to Ford and they replaced the unit - I'm the only person that has needed it apparently - but it still does it.

Has anyone else had a similar problem and could it be the tow bar electrics causing a short?

I'd be grateful for your responses.