First of all most professional car audio installers will have been certified at some point in their career. Any decent professional will have a certification; however, this does not necessarily mean they are really good.
An excellent installer will always keep his patience and will never rush a job. Find out what the average time of installation is for your car audio system and make sure he or she leaves enough time that is not too much less than the average. Ask as well if he is familiar with the installation process of your car audio system and how he would go about it.
Ask him further questions, which will indicate his level of competence. Inquire about his or her understanding of the laws of electricity, how he would make any alterations correctly and safely, and his understanding on the usage of tools. These all vital bits of knowledge any good car audio fitter would know.
Having the right tools is vital to this profession. Inquire exactly what tools the fitting may require. Any good fitter should be able to pretty much answer straight away, without too much hesitation.
Lastly, a good fitter will always keep up to date with the latest technology. Don't expect him to know straight away, as he may have to go away to research the product to complete the harder and more complex areas of installation. However, a good car audio fitter will always freely admit that he is not as familiar with the product as others (as you can't be fully familiar with everything). Inquire if he researches the newer products and the installation process for these.
To fit a car audio system correctly, you need to have quite a bit of knowledge and expertise, as well as patience and an ability to adapt to ever-changing technology. A good car audio fitter should be able to demonstrate all of these.
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