  Infrared reflection measurements of the half-filled two-dimensional organic
conductors $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_{2}$]Br$_{x}$Cl$_{1-x}$ were
performed as a function of temperature ($5 {\rm K}<T<300$ K) and
Br-substitution ($x=0%$, 40%, 73%, 85%, and 90%) in order to study the
metal-insulator transition. We can distinguish absorption processes due to
itinerant and localized charge carriers. The broad mid-infrared absorption has
two contributions: transitions between the two Hubbard bands and intradimer
excitations from the charges localized on the (BEDT-TTF)$_2$ dimer. Since the
latter couple to intramolecular vibrations of BEDT-TTF, the analysis of both
electronic and vibrational features provides a tool to disentangle these
contributions and to follow their temperature and electronic-correlations
dependence. Calculations based on the cluster model support our interpretation.
