I've got an old Schwinn dynamo light set in my parts box, and maybe it can be a helpful reference.
It uses the dynamo body, brackets, and the bike frame for the "ground" connection, which is a potential source of troubles.
The other connection is through a weird terminal in the bottom of the dynamo that is shaped like a cup, and relies on shoving in wire and hoping it contacts it. I think perhaps you are supposed to wrap the bare copper wire around the little post in the plastic plug that gets shoved into the cup? As someone who has worked with electrical connectors for decades, it is hard to think of a worse way to make a connection.

a couple of pics....

the light set:


the bottom of the dynamo, where the connection is made:


If you've got a connection like this, check it and see what can be done to minimize the effects of oxidation or corrosion, and perhaps see if the wires can be made to fit tightly against the metal terminal?

Steve in Peoria