Written by Randy at FRCN. Originally posted at allpar.com; adapted and used by permission. We do not work for and are in no way affiliated with any of the above mentioned companies. We cannot take any responsibility for any results consequences of the use or misuse of this information.
While many people wish to upgrade their stereo or put a modern stereo in a classic car, they don't want to tear up the dash and they don't want to attach brackets to hang things under the dash. Even in a modern car, people like to have a system they can transfer to their next car. These things are all possible. Here's are the basic items in one of these systems:
The best dollar-to-sound value system that can be installed today is based on a power amplifier. These 12 volt amplifiers put out very clean sound in a small package. They can be hidden in many places; under seats, in the trunk, or even in a door panel in some cars. They are available in a wide range of quality and power, but for most people an amp that is rated at about a total of 100 watts is fine (this would be about 50 per channel, or 20 watts per channel RMS at .5% distortion). If this system is going to installed in a 6 volt car I wouldn't go any larger than this. Also, amps with lower power rating are smaller and easier to hide. The amp should have remote turn on and low level inputs (most do).
This is the one area that must be evaluated subjectively. Generally, I like the 6x9 coaxial 3 way speakers. They have a good bass and don't take up a lot of room. In 6 volt applications, don't go for monsters rated at high wattage. These are sometimes not as efficient (they need more power to produce a given decibel level). With the above amp, speakers rated at around 1.5 times the amplifiers power rating or just above that should be O.K.
[Corolland additions] Speakers are fairly easy to install in the doors - once you get the doors apart. However, you must be sure to get shallow enough speakers to fit. Do not get cheap with speakers, they are probably the most important part of the sound system. You can pay $40 for a pair of name brand speakers at Pep Boys and get something that doesn't work as well as the stock system!
Modern electronics have brought us the personal/portable CD player. This should be your first choice. The S/N (signal to noise) ratio is excellent in the worst units. Many now are equipped with electronic anti-shock and some even have different equalizations built into help match the sound to the vehicle/speaker combination. The second choice would be a personal cassette player. These work well also, but of course, you can't expect the sound quality that a CD player can produce. I have installed three systems with cassette players and they sound just fine! Cheap CD players sound better than cheap cassette players.
There are now available small, solid state devices that can change 6 volt negative or positive ground into 12 volt negative ground, and 12 volt positive ground to 12 volt negative ground. In all these cases, the ground is common so isolating the amp or the speakers is unnecessary. The unit is very small (a cube of about 3"), and only has three wires- input, output, and ground. This can be mounted anywhere because of its small size.
Speakers are the most difficult to place. Later [1970s and beyond] cars may have a place under the package shelf in the trunk set up for speakers- this should be your first choice. Another location is under a seat mounted behind the skirt- this may muffle the sound a bit. I have mounted mine in small, carpeted enclosures commercially available and designed for this purpose. They have quick disconnects so they can be removed from the interior for car shows and security. [You can add these enclosures to stock speakers. Depending on your car, replacing the speakers on the deck lid can be incredibly easy or surprisingly hard. Get into the trunk when you do it, and by all means wear goggles! Stuff falls down.]
This should be in a dry environment where it will not be bumped, and have clear air movement around it to dissipate heat. You will have to decide because it will vary from vehicle to vehicle. Under the package shelf next to the speakers is a good place. Under the front seat is also good because the low level input wire will be shorter and less likely to pick up interference. Under the dash is also a possibility in larger cars.
A portable unit can go on the front seat next to the driver, or in the glove compartment. The power wire (if you are not using batteries) and the low level input to the amp go under the carpet and poke up between the seat cushions. This way, when you leave the car and take the CD player with you (or lock it in the trunk), you can tuck the wires into the cushions and no one can tell that the car has a stereo in it. The signal wire plugs into the headphone jack on the sound source and goes to the low level input on the amp. [The webmaster used to have one in the glove compartment, where it rested on a little cushion of its own. You can easily run the wires in through the back of the compartment and it will not be obvious from the outside.]
My car does not presently have a headliner in it, so I was able to run the wires through the roof channels from the trunk (where my amp is) to behind the front right kick panel. The important thing to remember is to run wires where they will not be abraded by foot traffic or caught in moving parts. Be sure that all power leads are fused and that you have a good ground for the amp. Do NOT attach the ground to the amp mounting screws. Use a separate screw that holds the ground only. Solder all connections (do not use crimp connectors). This is particularly important in 6 volt vehicles where every amp and volt counts! If you system is in a 6 volt car use large gauge wires rated for more than your amp. I used a Monster Wire kit designed for 500 watt amp installations. This limits power loss to a minimum.
A local auto sound shop would be the best place to start. There you can listen to different amp/speaker combinations. This is particularly important when it comes to speakers. I used J.C. Whitney and was able to get the amp, speakers, and power inverter on sale.
The power inverter will be the most difficult to find (if you need one). J.C. Whitney only carries the unit that inverts 6 or 12 volt positive ground to 12 volt negative ground. If your car is 6 volt negative ground contact: Antique Automobile Radio Inc. at 1-800-933-4926 for more information (they will send you a catalog). There are two of these units listed in Whitney- get the higher rated unit!
I got everything from J.C. Whitney [in 1996; you may also want to try AutoAccessories]. The Pyramid power amp was $38, the speakers were $38, the 6x9 speaker boxes were $18, the wire kit was $15, and the speaker wire was $7. This was about $116. If you already have a CD player or Cassette player, you are in business with a sound system for just over $100.
The bad news is that if you have a 6 volt system the power inverter will cost another $80 or more depending on where you get it. But even with this, the system will only run about $300 with a CD player, and about $250 if you but a decent cassette player.
Even with this modestly rated amplifier, there is more than enough sound output to please all but the most high-DB burned-out people. The sound is very clean and pleasant. I left the speaker wires long enough to be able to place the speakers up on the back seat while on the road, and on the floor when I park.