Whoever it was that first thought of putting radios and such in cars had a winning idea. Nowadays, you pretty much take for granted that you can cruise down the highway to your favorite music.

But like other consumer electronic fields, a lot of changes and innovations have come along in recent years. Radios have become stereos and tape players and much more. There are products that come factory-installed, there are systems you can add afterward.If you haven't been shopping for awhile or are contemplating a first purchase, you'll be surprised to find such a variety and range of quality in automotive electronics products. Integrated circuitry makes it possible to pack more features into smaller and smaller packages and at a variety of price ranges.

In shopping for auto electronic products, be sure you know what features you want and what you can afford. To help you decide, the Electronic Industries Association/Consumer Electronics Group answers some of the most commonly asked questions about auto electronics:

What constitutes a good system? It depends on how much money you wish to spend.

The most popular sound systems consist of an AM/FM stereo radio receiver, cassette player, power amplifier and anywhere from two to six speakers. You can also add a compact disc player. Cost can range anywhere from $250 to $1,500 and up, plus installation of $100 and more.

At least 10 and more often 20 watts per channel will give you enough volume to compete with the road noise and enough bass to satisfy most sensibilities. (As a safety precaution, however, never have the sound so high as to block out road noise such as a car horn, siren, etc.)

What about additional features? Depending on the sound system, such features as digital electronic tuning, seek and scan, 5-8 station preset tuning can be added. A preamp output will bypass the built-in amp and add clean sound from as powerful an amplifier as you decide to purchase. Other options include separate bass and treble controls, a fader to adjust the balance between the front and rear speakers; memory preset stations; Dolby noise reduction, auto reverse to automatically play both sides of the cassette tape and a tape equalizer switch to play normal bias or high bias tapes.

What about buying a car with the sound system already in place? It certainly saves time to buy a car with a factory-installed sound system. A factory-installed system also has the advantage of having been designed specifically for your vehicle.

Some car dealers also sell quality aftermarket sound systems. When ordering a new car, look carefully at the list of optional and standard equipment. While most mid-priced and luxury cars include some type of sound system as standard equipment, you can frequently upgrade the receiver, speakers or both.

Where else can I get a sound system? Auto sound specialty stores and consumer electronics stores carry a wide range of products as well. You may want to do some price checking to compare with factory-installed prices. Be sure to include installation fees and check on warranties.

How do I know how a system will perform? Any sound system demonstrated in a store will sound different in your car, which has a peculiar mix of sound-reflecting material (glass) and sound-absorbing material (upholstery). That along with the car's interior size, speaker placement and other factors will cause certain frequencies to be emphasized and others to be de-emphasized. Road noise also tends to mask high and low frequencies.

In a store or showroom, you'll be able to judge how well the receiver picks up weak and distant stations but not how susceptible a particular model is to "picket-fencing" (a rapidly fluctuating FM signal strength that creates a pulsing sound) or other noises created because of multipath interference.

In addition, while some cassette players can stand the bumps and jolts of a rough road better than others, a tape transport mechanism's speed is affected, producing a type of distortion known as wow and flutter.

What can be done to help this? The use of high-power amplifiers can boost treble and bass frequencies that might otherwise be lost due to road noise. You can install an equalizer to boost or attenuate certain bands of frequencies.

How do I know if a sound system will fit my dashboard? There are five difference chassis sizes for cassette-receivers. Some will fit perfectly in your dashboard, some will require minor modification and others will need major adjustments.

Retailers generally have a list of which units will fit in which cars.

Does it matter where I'll be driving the car? Yes. In rural areas, cassette-receivers with a higher sensitivity rating are your best bet to pick up weak and distant stations and still get noise-free sound.

In urban/suburban areas, however, you're likely to encounter reception problems caused by tall buildings and other obstructions blocking or reflecting the signals. Some receivers have refined circuitry and other features to deal with these problems. You'll also want a system that can deal effectively with front-end overload and that has a high measure of FM selectivity. (Front-end overload occurs when the signal from a nearby FM station intrudes on the weaker signal of a more-distant station. Selectivity refers to a receiver's ability to prevent a strong signal from overpowering a weaker one next to it on an FM band.)

What about AM stereo? Does it sound as good as FM stereo? Today AM as well as FM stations can broadcast in stereo, but you must have a special circuit to pick up an AM stereo broadcast. AM stereo offers some advantages. Because AM signals aren't directional, they won't suffer from picket-fencing, thus making AM stereo a good idea in areas with many tall buildings.

If your favorite stations are broadcasting AM stereo you may want to consider getting a receiver capable of receiving it.

Compact dics are coming on strong in home sound systems, what about in automobile systems? Compact disc players are now available for cars and provide many sonic listening advantages. Distortion is practically eliminated, background noise disappears, dynamic range is increased and frequency response is widened.

Because of advanced design, rough roads pose minimal threat to CD playing. Mistrack may occur only in the most extreme circumstances, and even then, the laser memorizes its position to instantly pick up where it left off.

Often CD players can be added on to an existing cassette-receiver system. There are a few models that combine AM/FM tuner, cassette deck and CD player in one package.

How many speakers should I buy and where should I place them? Some people are opting for four or even six speakers rather than two. With this many, the soundis equally dispersed between front and back passengers.

Front speakers can be placed in the front doors, in the top of the dash and in the front kick panels. Indoor speakers will provide more bass response than dash speakers.

Rear-deck speakers also provide better bass response because they usually have a large trunk cavity behind them.

Can I install my own sound system? With the complexity of circuitry found in most new cars today, it is nearly impossible for the average layman to tackle even the most simple installation.

The damage that can be caused to existing electrical systems more than warrants the consultation of an expert installer. Otherwise, you may not only jeopardize the factory warranty on your sound system but on your car as well.