Any idea where to get a fuse diagram for the outside lights?

it comes off the light switch
you did not tell us which chassis, what year

start tracing wires from the light switch
get the chiltons book for your chassis, and you should get the chassis wiring, light switch etc..

running/marker/clearance lights
there is the possibility of switch failure, but the fuse is in the chassis fuses
under the hood or under the dash