Make sure the power amp is capable of handling speaker level inputs; if it only has line inputs you may want to rig up some sort of voltage divider on the input
to attenuate the signal coming from the Blaupunkt. A lot will depend on how much
peak to peak signal the Blaupunkt is capable of swinging.
Worst case scenario would be a burned out input stage on the power amp; more likely would be distortion and/or an overly sensitive volume control.

As for the generator being adequate to handle the added equipment, you need to look at the total current consumption of the power amp. Take whatever the total RMS output per channel is, and double it, for a rough estimate of maximum current consumption.

4x25 watts RMS (not peak music power) = 100 watts, doubled = 200 watts x 13.6 volts DC gives a maximum current draw of 14.7 Amps. You may have trouble if you run lights and wipers/blower at the same time.

If the amplifier's power rating is listed as 25 watts peak, the real output is more like 12 watts RMS. If it is listed as Peak Music Power, the real output will be more like 8 Watts RMS. Car stereos makers are notorious for using non-standard terminology to make their equipment seem more powerful than it actually is.

Bill