Placing a diode between low and high beam on a motorcycle
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Addition of a relay.

As Spehro points out a relay solves this nicely. Provided you can run a few wires you can hide the relay anywhere.

Note that your switch contact may not handle 12.5 A well. To keep it running with only 8 A as supplied, wire the relay contact from the battery rather than the high-beam wire.