1992 chev silverado

Yes, you can have both of these units in your truck. The only problem you might run into is that one of these two units must have AUX input. This will allow you to have sound when you are choosing between which unit to play.    You can not wire both of these units with two sets of speaker wire running into the same speaker. In the case where both of your units don’t have AUX input you can use a FM modulator and a line output converter to get the sound. This will allow you to convert one of the units speaker wire signals to a FM station signal. One of your units will pick the audio sound from the other unit on a preset FM station. If you are spending the time and money to have these two options that you want in your truck. (CD & TAPE). I think the better option is just to go out and spend your money on a new unit with the option that you want.   You will probably have a better sound system and less the potential headache from all the wiring you got to do.

p.n.

"So if I have an AUX port in my CD player, I can, say, tie in my cassette output, and run my speaker through the CD output only?"

Yes precisely!

"If possible, could I splice my CD's output speaker wire individually into the cassette individual speaker wire outputs, or will my signal be interfered with?"

You would not need to if you have the speaker wire running from your CD unit and not the other way around. (speaker wire to cassette unit) With this set up all you do is select AUX if you want to listen to music on your cassette unit. Is works the same way as if you were listening to music on the radio station. You would press TUNER to get the radio station , AUX to get music on the cassette and CD for music from the CD unit itself.