Simple, just use a small gauge wire, connect one end to one of the + wires on
one of the Fog Lights. Then the other end to + side of the instrument bulb you want
to light when Fogs are on. Be sure that that instrument bulb is grounded from its other wire. DONE.
When power is applied to fog lights, it's also applied to the instrument bulb lighting
it as well. Fogs on, instrument on, fogs off, instrument off.