Just guessing here, since I don't know the model number of your head unit or the amp, that it's 25 watts per channel, 4 channels; that's the typical setup. Check your speakers--if they are marked "8 ohms" on the back, they are wired in parallel sets to allow for 8 speakers on 4 channels. Be verey careful about replacing an 8 ohm speaker with a 4-ohm aftermarket. Use the search function at the bottom right for fuller discussions. You might also have a unit which runs the dash speakers at about 15 watts max, and four additional channels, with one pair of channels haveing 8-ohm parallels.
Good luck.
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We have met the enemy and they is us. [Pogo] '99 S70 cop car : Rough Rider tires& suspension, Walmart fog lights, speakers by ebay, ambiance by Pall Mall, trim by Le Duc d'Tape