This theory has not worked so well in practice, primarily because manufacturers have not made the system fool-proof by creating a database of legitimate owners, or even a register of stolen radios. If a car fitted with a radio is sold on, there is no record of who owns the radio.
Tony Hutchinson, a partner in Square Red, a design company, bought his pounds 160 Blaupunkt Verona CR31, a cassette-radio that can be slipped in and out of the car and carried like a handbag, because of its security coding.
'It comes with its passport book and the relevant documents and information on what to do if it is stolen, such as reporting to the police the code and serial number, and what to do if you have lost or forgotten the code number; how to send the radio back to the factory to get it recoded.
'Every time you put the radio back into its slot, you had to enter the four-figure digits before you could use the radio.
'Well, the radio was stolen recently, and I phoned the police, who were not the slightest bit interested in its security code, because they said the security code had no value.
'I rang Blaupunkt to inform them that the radio had been stolen. I was told that they were not interested either, as they did not keep details of codes. The recoding element was sub-contracted to a private company.
'I rang the company, Contel Communications, and asked if they had a list of stolen radios and their codes, but they said no. I asked what would happen if, as a thief, I sent in a machine to find out its code number, with some excuse. They said they would simply send it back with the right number and would charge a standard fee of pounds 35 plus VAT.'
Paul Salzedo, Contel's service manager, said: 'We do have a list of sets which have been stolen and which different dealers have informed us of, and we do get inquiries from some police stations, but it is not very comprehensive.
'The problem is that when these radios first came out some years ago - and most manufacturers now do them - it was easy to have a record of who had bought them and demand proof of purchase. But as ownership has diversified, it has become impossible to keep a track of who owns every radio or which one has been stolen.
'Anyway, a lot of the youngsters who steal radios, just pass them on down the line for a tenner and don't care whether they are useable or not. Better systems are now being developed, such as using smart cards, which will eventually outdate security coding.
'Reports that security codes can be erased by putting the radios in the deep freeze are nonsense,' said Mr Salzedo. 'All that does is ruin the radio. I think the theory must have been put up by the first thief who stole one of these radios, to encourage someone to buy it.'
Security coding increases the price of the radio - pounds 50 is about the starting-point - but is of no interest yet to insurance companies.
A spokesman for Guardian said: 'We do monitor claims, and if it started to show up that radios which were security-coded were being stolen less, then we might think about giving a discount for them.'
Direct Line said it also might be looking at discounts 'in the future'.
Richard Lenthall, the product manager of Blaupunkt, said: 'It is down to the dealers to ask for satisfactory proof of ownership. Contel do have a computer, on which every radio they handle is logged. And it should flag anything if it is returned quickly or too often.
'On our more expensive ranges, we also input the car registration number. It is not so much the money which stops us having a database, but the lack of practicality. It is impossible to keep track of where the radios are. We are reliant on our dealer network.'
Mr Hutchinson said he would not buy another security- coded radio and would certainly not put the stickers saying 'This car has a security-coded radio' on his vehicle again. 'All you are doing is telling thieves that you have an expensive radio on board,' he said.
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