Okay, let me start off here by saying, it's hard to beat the factory Delco ETR radio that comes in most GM cars. Even the older ones had an 80 watt, 4 channel amplifier. Some even have a built in 5 band equalizer and music searching tape deck. There is something about the matching look of these units that is hard to beat in a GM car. With changes made to other (hidden) parts of the sound system, you can obtain excellent sound quality from this radio.
Start off with the speakers, if the car is old, the moldy, dryrotted speakers must go! The rear 6x9's can be replaced with inexpensive ones, the $13 Audiovox speakers from AutoZone come to mind. Up front it is important to use coaxial speakers with crossovers to eliminate some bass distortion from the front speakers. The coaxial tweeters will give you that high end clarity the factory speakers are missing.
About the only thing that fails in these radios is the tape deck drivebelt. This can be obtained from any electronics repair facility that orders Delco parts. If a tape is stuck in the radio or it switches sides constantly this is the problem.
The most desirable version of the ETR has the graphic EQ with Music Search tape deck. With this and a good set of speakers, you will have a very enjoyable stereo.
But what if you want a CD changer? Easy. There are 2 ways to interface it with the radio. The first and simplest is to use the RF modulator that comes with the CD changer. This will necesitate you set one preset button to a specific frequency for the CD player. The signal comes in through the antenna and is good quality, but not as clear or powerful as it can be. Plus you have to hit a preset button to listen to the CD.
The second method involves some fairly simple modifications of the radio, and maybe the CD changer modulator. In effect this allows the CD changer to send its audio signal into the radio's EQ and amplifier directly without going through the RF modulator or reciever/ tuner. The CD audio quality is as spectacular as you expect CD audio to be. Unlike the RF modulator system, the CD audio can have its full range of amplitude. The quiet, soft sounds are super clear, followed by thunderous bass with no loss of clarity, hiss, or other distortion. Also the radio will automatically turn on the CD input mode when you turn ON the CD player.
This is accomplished by "fooling" the radio chassis into thinking you have put a tape in the tape player, then sending in the CD player's signal where the tape player signal would go in. The aux input relay changes the signals in the tape player wiring to accomplish this. See diagrams. The chassis then turns off the tuner, and expects audio input from the tape deck. The aux input relay also disconnects the tape player audio-in wires from the tape deck and reconnects them to the line-out from the CD player. The engineers at Delco Electronics were nice enough to use normal audio-level signals in the radio, so a standard CD changer will sound at the same volume as the tuner and tape deck. The relay I used came from radio shack, it a Siemens "ice cube" 4 pole double throw 12Vdc coil unit. It will fit inside the radio beside the tape deck and be completely concealed. Only the RCA pigtails for input, and the "aux activate" wire emerge outside the radio.
I have a Profile CD player, with a wired remote. It has a "modulator" box with the 2 audio output RCA cables coming from it, mounted in the dash near the radio. I opened it up, and found a point on its circuit board that goes to +12 volts when the power button on the CD changer control is pressed. It is what powers the remote control display backlighting. This signal powers the auxiliary relay in the radio, thereby kicking the radio into aux mode whenever the CD player is turned on. If you can't figure out how to add this to your CD player, a smple switch that applies +12V to the relay will do, but then you loose the "fully automatic" control.
Connect the "ignition switched power" lead of the CD player to the pink "power antenna" wire of the radio and it will allow the CD changer to only come on when the radio is on.
Some points I discovered while expirimenting with these:
In the standard tape deck version, if you don't disconnect the red/ white wire with the auxiliary relay, it will mix the radio and CD player audio...
In the Blaupunkt tape deck version, do not use either of the black wires as ground for the relay coil. Doing so will "lock up" the radio's CPU and you will have to turn off all power for a minute to get it working again...
The CD player must be removed form the circuit when in normal (not aux) mode. If it is left in the audio circuit (even inactive) the tape audio is degraded. In my diagrams, the relay does this function...
In the standard tape player version, if you don't have your relay turn off the power to the tape deck (red wire), the tape motor will run when power is put on the brown wire, and the tape player will click constantly if no tape is in it.
If there is a loud "pop" in the system when the CD player is changed from play to pause, or switched track to track, insert a 100mF capacitor in series with each CD player line input...
This is the Blaupunkt tape system. In this system, the tape player grounds the yellow wire when a tape in inserted to activate tape mode. The relay turns off power (red wire) to the tape deck and grounds the yellow wire.
This is the standard tape deck. In this system, with no tape in the tape player sends +12V into the chassis on the red/ wht wire to make the radio play, and when a tape is inserted, it sends +12V in on the brown wire to activate tape mode.
The relay disconnects the red/ wht wire (turn OFF radio) and disconnects the red wire (remove power from tape deck) and connects power to the brown wire (activate chassis into tape mode).