We have made measurements of the local potential distribution and topography of current carrying Y1Ba2Cu3O6.9 thin films using a scanning tunneling microscope. We have compared the electrostatic potential distribution in the normal state on post-annealed thin films and in-situ prepared films. On post-annealed films we find steps in the potential image indicative of insulating boundaries in the material which occur at boundaries between clusters of grains whereas the in-situ films show a nearly uniform potential drop. These results provide a clear indication of the nature of disorder in these thin films and what length scale ultimately determines the microscopic resistivity.