Amp conex, how much is too much?

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OK, being a noob to this forum take it easy on me. I was curious and unable to come up with the answer to my question so I have come here.

My '02 Land Rover has a factory Harmon Kardon amplifier that drives everything. The receiver will not work with out it and the connection is a large wiring harness (I don't know which wires are which). Can I simply tap into the lines connected to the little woofers mounted in the rear door and mount a small amp to drive an 8" or 10"? How much is too much power to attach to the inputs of an amplifier? Or is any power too much to attach? Is there a way to measure the amount of power going to these very small speakers?

TIA,

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