Thanks to all of you that's tried to help me with this but I'm giving up and buying an ugly device to stick on my windscreen.
That's now three bought and three home-made microphones, all getting the same results. I'm thinking it may be the DVR itself that's the problem, or part of it. The DVR is a 12v unit but maybe it was never designed to be fitted into a vehicle.
This is going to sound awfully controvertial, so no offence Andy!
People try to help in their own way!, usually people respond with a solution that is un-tested. You responded with a typical "tried this, tried that!" And "it didn't work"! Then 'Gave Up'.
What didn't work?
How did you make the circuit I linked to ?
How did you power it?
How did you check it does'nt work?
Also, it's probably line-level needed for the DVR which we would have done in the next stage...
But without feedback, how can we help you?
In hindsight, perhaps I could have asked better questions. But, Martin has a valid point.
In electronics things must be done methodically.
When quick assumptions are made, mistakes are made.
People here are great to volunteer their time and knowledge, but the onus is on the op to give adequate feedback, and make sure they completely understand.
I don't think anyone involved wants to waist their time or money on something fruitless.