if you want to go Audio Store and want to buy some Video Audio Unit you must read this step : 

Read car audio magazines such as “Car Audio & Electronics” and “Auto Sound & Security.” They will tell you which products are available and what features they have.

Make a prioritized list of the features you want in a stereo.

Write down the make, model and year of your car

Get a cassette or CD of your favorite music

and at the Audio Store

Ask a salesperson to suggest stereos based on your prioritized feature list. If the choices bust your budget, eliminate features one by one until you find at least three stereos you can afford.

Listen to each stereo, using your own music, and select the one that sounds best.

Make sure your choice is easy to operate.

Check that everything you need is in the stereo’s packing box. The package should include a faceplate, carrying case, sleeve, main plugs (some have one, some have two), one or two metal keys (for removing the stereo), an owner’s manual and an installation manual.