If it's a battery light and you're running it in flash mode, don't do that. Run the light in the brightest mode it's got, so you don't have any current switching going on, which would also generate magnetic fields.

If it's a dyno light, you might want to look into replacing the light wire(s) with coax cables, like the SON wires. The coax might provide enough shielding to let the speedo work.