  We investigate the dynamics of a single phonon (oscillator) mode linearly
coupled to an electronic few-level system in contact with external particle
reservoirs (leads). A stationary electronic current through the system
generates non-trivial dynamical behaviour of the oscillator. Using
Feynman-Vernon influence functional theory, we derive a Langevin equation for
the oscillator trajectory that is non-perturbative in the system-leads coupling
and from which we extract effective oscillator potentials and friction
coefficients. For the two simplest cases of a single and two coupled electronic
levels, we discuss various regimes of the oscillator dynamics.
