After you have wired the dimmer correctly FOR SURE.

1. Unplug each headlight plug. Let them hang.
2. Use a ice pick test light.
3. Attach test light alligator clip to a good ground place.
4. Turn on headlight switch.
5. Probe each hanging plug and see if you get voltage on the same plug contact on each plug. One matched set of plug contacts should show voltage. Then click dimmer and another matched set should show voltage.

You are testing the disconnected plug, not the light bulb.

One of the tests will show nothing because it's the ground contact. Look at the pic I posted for ref. Note it's the back side of bulb. But you just need to know that the voltage is at the same contact on each plug. At each click of the dimmer.

If not, that needs fixed.

If the terminal block has been deleted by PO, Then he probably spliced those wires from the dimmer so as to split each wire to left and right. That is where you need to un-tape/unsplice and fix.

Post any questions at any time.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn