The 45W is peak power per channel. Stereo manufacturers rate head units with peak power to make them look like they are soemthing they are not. Average power is probably more like 15-20W (of dirty power) which will almost certainly be fine with the factory speakers. Factory head unit is probably putting out about 10W nominal. On the bright side it will be an excuse to get good speakers later if you destroy them!

One thing to note is that with a speaker it is better to over power it than under power it unless it's a subwoofer. More damage is done to speakers from not getting enough power which causes signal clipping. This is not a problem on subwoofers and generally the opposite is true.