  We formulate and apply a nonperturbative numerical approach to the
nonequilibrium current, $I(V)$, through a voltage-biased molecular conductor.
We focus on a single electronic level coupled to an unequilibrated vibration
mode (Anderson-Holstein model), which can be mapped to an effective three-state
problem. Performing an iterative summation of real-time path integral (ISPI)
expressions, we accurately reproduce known analytical results in three
different limits. We then study the crossover regime between those limits and
show that the Franck-Condon blockade persists in the quantum-coherent
low-temperature limit, with a nonequilibrium smearing of step features in the
$IV$ curve.
