If you have the posh stereo, that's the Harmon Kardon speakers and Clarion (early) Alpine (later) stereo install, they operate on balanced line level outputs to separate door speaker amps. It reduced interference, and makes the sound quality brilliant... but it also means it won't play nice with many off the shelf head units. You can get a lot of noise, buzzing and hum if you try and just replace the unit without any modification to the wiring. It's to do with plugging an amplified signal in to another amp. People have made little looms to attenuate the signal (attahced below). But what you really need to do it provide balanced audio to your door amplifiers, or skip the amps all together, and just wire your speakers in to the back of your new head unit.
If you have the standard stereo in your car, then none of this applies!!!
The attached is how
@Clarky130 did it. I'm not surer he's on here any more.....