  Transport through coupled quantum dots in a phonon bath is studied using the
recently developed real-time renormalization-group method. Thereby, the problem
can be treated beyond perturbation theory regarding the complete interaction. A
reliable solution for the stationary tunnel current is obtained for the case of
moderately strong couplings of the dots to the leads and to the phonon bath.
Any other parameter is arbitrary, and the complete electron-phonon interaction
is taken into account. Experimental results are quantitatively reproduced by
taking into account a finite extension of the wavefunctions within the dots.
Its dependence on the energy difference between the dots is derived.
