Dash units and Amplifiers

[tf95ZR1]

I have had cars where each speaker located about the cabin
had just (2) wires to connect: a (+) and a (-) wire.
The head unit in the dash contained the amplifier(s).

I know that our speakers each have an accompanying amplifier
located within the speaker enclosure.

Would any dash unit still work in our cars? Would they
effectively be "double amplified"? Just trying to figure
it all out. TIA!

Ted