1999-2003 7.3L Powerstroke gauge lighting power source.  Arrow is pointing to a light blue wire with red trace.  To remove the headlight switch, pry the instrument cluster (dash front) out on the left side just enough to gain access to the headlight switch module.  With a flat blade screwdriver, push in on the metal retaining tab on the left.  Then do the same for the 2 remaining tabs, one on top and the other on the bottom of the module.  If you desire you can tap into the same wire located under the dash.  It is explained on the previous page for the 6.0L Powerstroke.
NOTE: There are many different locations where you can obtain your dash illumination source. Depicted above is just one example.  Irregardless of where you connect your gauge power and illumination source, we recommend that you check these sources first, before you make your connections.  For Dash Illumination: Using a voltmeter, ground the black lead from the meter to chassis ground. With the red lead off the meter, probe the read lead of your voltmeter probe the suspected dash illumination source.  With the running lights off you should not read any voltage.  When you turn the running light on you should see the voltage rise and fall with the adjustment of the dash lighting.