The next time a stereo enthusiast invites you over to hear his new rig, you might end up in the garage listening to his car.

While the home-stereo market is in a slump, auto sound sales are booming. Car-owners are filling their dashes with receivers and tape decks; their trunks with pre-amplifiers, amplifiers, graphic equalizers and other components; and their rear decks, doors and dashes with an assortment of speakers. Like homeowners who, unable to afford a new house, are turning to extensive remodeling, car-owners are now equipping their five- year-old models with the latest in auto sound equipment. Altec Lansing, JBL, Sony and Sansui are breaking into this market long held by such traditional auto sound manufacturers as Jensen.

Although a good system can cost as little as $300, an elaborate, top-quality system can easily cost $3,000 to $6,000, with installation costing just as much. A well-equipped car can now cost as much as a small house did in the 1960s.

Auto components' specifications and features can differ little from a good home stereo system. An auto speaker system can include subwoofers, woofers, midranges and tweeters, driven by an amplifier delivering as much as 50 watts per channel.

Even mid-priced auto units now typically include noise-reduction systems, automatic reverse, metal-tape capability and electronic tuning. The high-end units include automatic program search and built-in graphic equalizers. Fujitsu Ten even offers remote control, so that anyone in a vehicle can control the cassette deck or receiver program.

As with home stereo systems, personal computers and other technological products, prices of auto sound systems are dropping; it's just the additional features, components and speakers -- and the top- of-the-line quality once reserved for home systems -- that make it possible to spend so much more.

The amount of available space, interior materials, vibrations, noise and changing station signals -- all add to the demand for special features in auto components and for special efforts in installation.

Digitally synthesized tuning, which many auto receivers offer, permits station pre-selection and scanning of broadcast bands until a strong signal is reached. Some manufacturers, including Kenwood, Mitsubishi and Sanyo, use sophisticated circuitry that will reject competing station interference. Kenwood's ABSS model receiver can be programed to either tune in another station or switch to cassette playback when the signals of a station begin to fade. Panasonic's Supreme Series features circuitry that maintains an FM signal as long as possible, switching to mono when the stereo signal is too weak. Jensen's new JR110 cassette-receiver features automatic local/distance switching. Engine noise and interference present more problems, and more receivers are coming equipped with a noise-suppression capability. Sony's in-dash receivers, for example, have a high filter to reduce noise and interference. As cars get smaller, receiver and cassette units are also getting smaller. Aiwa, Autotek, Jensen, Sanyo, Sony, Mitsubishi and Metrosound are among the major brands that have cassette-receivers with mini chassis to fit most imported and subcompact cars. Smaller and lighter cars also create installation problems for speaker systems. Bass sounds cause plastic interiors to buzz; so foam must be installed behind the plastic. While hard interiors created too much resonance in the past, interiors today are too absorbent. This absorbent material and the small interior space crimp the bass resonance, which must be boosted. But since the bass is omni-directional, woofers (bass speakers) can be placed anywhere -- most commonly behind the rear seat. Other speakers -- midranges and tweeters -- are normally placed in the doors and in or under the dash, although they too can be placed in the rear deck. Sealed enclosures behind the speakers are now being built by Sony, Mitsubishi and Visonik. Today's slimmer speakers can be put in the most shallow doors. Rear-enclosed door speakers by Altec Lansing, AudiSource, Visonik and others require less than two inches of depth. Upgrading speaker systems is the least expensive way to improve a car stereo system. But more car-owners are adding trunks full of components such as separate amplifiers for each pair of speakers, electronic crossovers, pre-amps, graphic equalizers and more. Some rival the most elaborate home- stereo systems. When car-makers talk about trunk capacity in cars of the future, instead of listing "one overnight bag, two suitcases, one two- suiter," they may well be saying,"three amps, one crossover, one equalizer."