The relay is rated to 30 or 40 A as a maximum current through the contacts. The coil is designed for DC and is rated for 12 V. It will take around 100 mA to run the coil.
As the headlight is run from the alternator directly, it is likely to be run from AC, and that is what is making the relay flicker.
You can use those very cheap diodes, and you would need 4 arranged as a bridge rectifier, so it would be easier to buy something like **broken link removed**
There are 4 wires, labelled +, -, ~ and ~.
Connect ~ and ~ to either side of the headlight bulb.
Connect + and - to the relay coil.