I'm actually mad that I threw out the AM/FM stereo-eight track from my '73 riv. I see it's worth quite a bit on ebay!
What I'll describe first takes a bit of expertise. The second option is so easy, almost anyone can do it.
before the introduction to high-fidelity/low-noise mobile dedicated pre-amps (A few companies started by just making processors), some people took their stock AM or FM radio, gutted the insides -- except for the lighted dial. A control center was made and fab'ed to fit inside, with stock pushbuttons having different functions: source selection, track <<, <, >,>>, stop, pause and a volume knob.
That approach is actually fairly tedious, because you have to tear the new source unit apart, get its wiring diagram, and use the mechanical trigger of a radio pushbutton to correspond to the new unit's operation with relays or solenoids. That was done in the '80's for some purists who don't want their old car altered in appearance in anyway whatsoever.
Simplicity, again is to do the JL Audio "clean sweep" which takes the speaker-level inputs of the old radio, and has inputs for tape, CD, DVD, DAT, etc.--even digital inputs for a DVD or CD or DAT (no one uses digital audio tape anymore; maybe just me)
ironically the tape function is going to cost you $$$$. Most of the remaining tape units worth selling are Nakamichi--which are indisputably the best tape decks ( very, very few thing you can make that statement about)in their TD-500,-700, 1000 series.
I would assume most people with tapes and vinyl would convert them to CD-R or DVD-/+R. Almost anyone's computer can do that with an inexpensive soundcard, or most of the time pre-installed on a new computer.
Unless you're a purist, an analog aficionado, or just want your vinyl to sound like vinyl, a hideaway CD changer or Sirius receiver,or even a portable tape player Sony pro D6 or D7-10) Walkman DAT's can all plug into the cleansweep.
With its auto-set-up and the cleansweep's32 bit DSP, one just uses a microphone in a seating position,and it will automatically adjust EQ settings/time alignment for that position.
The reason i write about this unit so much is because it takes all the guess work/hardship out of tearing out your dash, installing something else--which will ruin the aesthetics of beautiful Buick dashes-- look at some of those beautiful old '50's radios--, and then having to deal with the lengthy process of tuning a stereo (no, bass and treble controls are nowhere nearly sufficient), and having to have some source-switching device to select CD, tape, radio.
This JL Audio clean sweep is the first processor to completely address all the issues of old car owners (or new car owners with integrated climate control/radio/tape/CD, which are all in a non- DIN (8" x 2" rectangular opening), who for the former, want to preserve the cosmetic integrity of the dash, and for the latter, to have a means to add an aftermarket system
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Another cheaper alternative is a/d/s'642ix/642csi--which takes in speaker level outputs from your stock radio,and converts it to pre-amp level--allowing you to add any amp you want, and any SOURCES you want--which all remain hidden
On ebay, check out a/d/s to get some deals on these (~$20-30).
Stay away from FM modulators. why buy a digital source unit that transmits it to a radio frequency, which is received by your radio antenna? I always thought that was a convoluted way of introducing digital into old cars. The resulting sound will have less substantially less quality than the original digital source.
The hardest part of this is the labor of tearing your car apart. generally the trunk is where most audio electronic go, and wiring can quickly become a mess, if you don't label everything.
I finished the wiring for my '73 riv. Now I've got some work tomorrow making a template for the 1/2" plywood with a mass-dampening layer that will be bolted on top of the rear torpedo package shelf. Underneath the rear deck there are two factory 6x9 speakers holes WITH mounting nuts and bolts. All I have to do is make an MDF 6x9 template, cut out holes in that for component speakers,mark the bolt locations, use some more mass-dampening material on the mouting surface. I can fit a 6 1/2" woofer and 1" tweeter--which will soundbetter--and even handle more power than most 6x9, which are a big compromise.
This is a lot of work just searching parts--like getting a material that looks like the stock carpeting, which allows sound to go through; or perforated vinyl; or speaker grill material.
I don't want to sound snobby, but one thing recently deflated my impression of beautiful Buicks. The owner had FOUR 6x9 speakers with gaudy grilles on the package self.
Sonically, it's wrong--speaker should remain up front; although sometimes--like my Boattail, it's easier to use existing 6x9 holes in back, and not having anything showing on the inside.
(on my '63 there were no speakerson the package shelf-except for one between the rear bucket seats. I had to cut 7" holes for 2 subs and a 5" hole in the lowest door panel location possible. Just saying that I know will freak people out. The whole deck was reinforced, silicone sealed, covered in three layers of dynamat-like stuff).
And cosmetically, it looks tacky--especially with that high-tech Grey/silver plastic speaker grilles showing.