The Blaupunkt should be able to drive whatever speakers are in that car. Even mid-efficiency speakers with a sensitivity of 91db can be driven to 108db with 45 watts. Subtract 3db if speakers are 4 ohms, another three if they are 2 ohms. Most listening is done at under 5 watts. I don't see any reason for elaborate wiring. 2V from the preamp out is normal and should work unless the factory preamp is passive, ie., attenuation but no actual amplification. You do not need two preamps anyway so toss the factory one. I have a question, can you run the RCA into the power amps? Also, do the power amps have a gain switch?

Kuan