THEY SAY that reputations take years to build and days to ruin. But "they" are wrong - at least in the case of cars.

The car world is full of "prestige" makers who have committed embarrassing cock-ups.

And yet these manufacturers continue to bask in the sun-blessed glory of brand excellence and, in keeping with these reputations, charge exorbitant prices for their goods - goods that the punters, who choose to overlook most of these misjudgements rather than being blind to them, are happy to pay.

The reputations of the very top names in the car-manufacturing game - Rolls-Royce, Jaguar and Mercedes-Benz - have taken such a mauling in the past few years that you wonder why they have any reputation left at all.

Rolls-Royce once built the best car in the world, the Silver Ghost. It happened eight years before the outbreak of the First World War, which is rather a long time ago. Since then, it has never been true. Yet the Rolls-Royce myth continues. It has even become part of our language, a byword for exceptional quality.

Mercedes-Benz, by comparison, is one of the world's most technically proficient makers. It has made good cars for many years, never mind that they've had price tags to match. Its latest E-class and S-class cars, the big models which have always been the bedrock of the brand, are very fine and a cut above anything that comes from the mass makers.

But there are powerful signs that the powerhouse of the German car industry is beginning to lose its way. I like the new baby A-class and admire its freshness of thought. But its well documented propensity to roll over when challenged by Swedish elks was not just a huge public embarrassment, it also showed up a hurried and inadequate development programme which is contrary to Mercedes' key brand quality - thorough engineering.

Less praiseworthy than the unevenly developed but novel A-class is the basic C-class model, the C180. This "traditional" small Mercedes competes with the likes of upper-range Mondeos and is a car whose sole virtue is its prestige badge. It is unrefined and poorly equipped. At the end of the day it is little more than a pounds 20,000 Stuttgart minicab.

Further evidence of Mercedes' recent discomfort comes from America, where it makes the new M-class off-roader - and, by many accounts, doesn't make it very well.

Finally, on a business level, its new Smart baby car, on sale in Europe next month, has received the worst press reviews of any car in recent memory. Why buy a two-seater tiny tot with poor handling, when a four- seater conventional small car costs less and is better?

The Smart looks like being a dumb move for Mercedes.

Yet has Mercedes lost any of its cachet, after this cache of cock-ups? It has not. More than ever, people will pay over the odds to get a star on their bonnet.

Jaguar is very different from Mercedes. It has barely put a paw wrong over the past three or four years, under the careful guidance of its Ford managers.

Yet, from about 1968, when Jaguar became part of British Leyland, to the early Nineties, it did little right. Owners cursed as their Jags spent another afternoon on the hard shoulder waiting for the RAC.

Other owners swore as bits fell off and the electrics blacked out. Yet Jaguar has the most brilliant brand name! While the quality was appalling, the cars still looked beautiful and inspired such emotion. People will excuse a lot when they're in love.

I could go on. Land Rover is the world's best 4x4 name, yet the Land Rover Discovery has the worst reliability record of any 4x4.

And there have been loads of dud Volvos (usually small ones).

It just goes to show that brand names are often about as true to reality as the very industry which helped to promulgate them - the advertising business. And that, far from being sensitive to public moods and bad publicity, car makers are as cushioned from common sense as are those whose business it is to nurture them.