Leon W,
Is this light you want to fit a flash beacon, if so Then Yale specifye a dedicated connection.
There are some dedicated aux fuses located forward of the deadman line contractor check to see if one of these becomes live when accelerator pedal pressed, I certainly would not connect up any other way on this truck as I'm sure Yale specify a dedicate connection infact I would not connect any aux unless you know the correct connections to make.

Regards Titus