headlights always on high beams

Someone has hacked the wiring, or the harness has an internal melt, or otherwise "crossed up." Either that or your "new" dimmer is also defective.

Power to the headlight switch for ONLY the lights comes off the black ammeter wire NOT FUSED.

Through the HL switch and down to the DIMMER switch. From there hi and lo is selected, fed out through the bulkhead connector, around the harness to the rad support and to the headlights.