Hopefully, this info won't conflict with the article Brett is going to post and create confusion but this is what I did.
As has been stated in the post on the forum, old stereos used 8-10 ohm speakers and new cd players/stereos use 4 ohm speakers. Not exactly sure when they switched to 4 ohm. If you use 4 ohm speakers with an old 8-10 ohm stereo, it will work and will actually be louder than if hooked to a new stereo(that uses 4 ohm speakers). This because the 4 ohm speakers have less resistance. That's what the ohms are, resistance. It will be louder because the 4 ohm speaker has less resistance but that means that the stereo is moving more current than it was designed to move. As a result, if used this way frequently, it could burn up the amps in the stereo. Probably only a risk at high volumes but the service manuals have warnings stating that you need to use the correct ohm speakers.
If you want to use both stereos frequently, you would need to make sure somehow that, when the old stereo is connected to the new 4 ohm speakers, there is an additional 4-6 ohms of resistance in the speaker line to make it appear(to the old stereo) to have 8-10 ohm speakers like the original speakers it was designed to run. Could be as simple as putting a 6 ohm resistor in line with the speaker between the toggle switch and old stereo.
If you are only going to use the old stereo occaisionally and for a few minutes at a time, you probably won't even have a problem running the 4 ohm speakers directly to the old stereo. Just don't turn the volume way up. That's when I think you would run the most risk of over driving the amps in the old stereo. I didn't bother with the added resistor in my 442 but, as I said, I hardly ever play it unless someone wants to hear it at a show/cruise etc.
I also have a 78 trans am and installed two toggle switches in it to share the speakers between the new cd player and the factory am/fm/8 track. It's been a while since I did it but I think, while the factory ta stereo still used 8-10 ohm speakers, it does have a "return" line/ground like the new stereos instead of the speaker just going directly to chassis ground right at the speaker as it does on my 70 442.
So you just need two switches that have one set of contacts (two pins) on one side that control two pairs of pins on the other side. I think they are called double pole/double throw switches. On one side, you have two contacts I'll call the main contacts(which go to the speakers). One the other side, you have two pairs of contacts which I will call the secondary contacts 1 and 2. One pair of contacts/pins(1) will go to the old stereo's speaker outputs and the other pair (2) will go to the new stereo's speaker outputs.
So when the switch is in one position, the secondary pair of contacts 1 go to the old stereo speaker output and ground/return connecting the old stereo to the speakers.
When the switch is in the other position, the secondary pair of contacts 2, go to the new stereo's speaker outputs and ground/return.
On the TA, I drilled a couple holes in the left hand side of my glove box insert and installed the toggle switches there.
You could accomplish the same result with one double pole quadruple pole switch but that may be hard to find.
And for the electronics guys out there, I know there is a difference between resistance and impedance which is what the ohms rating on the speakers refers to but I was trying to keep it simple.
