@ericgibbs: Yes, that's the problem, however, the four motors (which are for the electric windows in a car, btw.) have four wires to them, and one common wire to return to the control unit. That of course means that all motors can only run in one direction at a time, but that is a restriction I can live with.
In fact, I think what I need is an amplifier with a gain < 1, so that the input voltage is actually reduced, so that +12 - Vs will give me approx. 5V (or let's say, +14.4 Volts, since this is the highest voltage I can get in a car.) I have tried a simple voltage divider for this purpose, but that just doesn't seem to work... I always read +12V - Vs at the mid tap, where I should actually read much less than that.
@Hero: I need to know each individual current drain from every motor. If I would put the sensing resistor before the relays, I could only know how much all motors drain together, but not for every single one.