But, when I connect the RioCar to the same power source as the cassette player, the voltage drops down to 5.5V.

What is the voltge at the cassette player before you connect the RioCar?

Is it possible that your Deux Cheveaux is old enough that it is equipped with a 6-volt system instead of the more contemporary 12-volt?

I envy you your 2-CV. I would love to have one, but they are next to impossible to acquire in the U.S. as they don't meet any kind of safety or emissions requirements, and the few really old ones that are "grandfathered" in sell for preposterous prices (approaching $10,000 if you can believe it!).

tanstaafl.