I had a similar experience, found it to be a mouse had set up home under the dashboard. The little sod then stored the poison bait I put down in the heater box, under the carpets, behind the A posts and in the heater pipes, interesting when I demisted the windscreen I got covered in the stuff, and even when I resulted in using the traditional mouse traps managed to get the bait off without springing the trap. The only way I could get him was locking the next door neighbours cat in the garage for a week with no food and water!
So thats what they do in Dorset - I shot mine with a 12 bore
