Inelastic electron tunneling spectroscopy (IETS) combined with scanning tunneling microscopy (STM) allows the application of vibrational analysis to a single molecule, but, up to now, IETS was restricted to small species due to the complexity of vibration spectra for larger molecules. The horizon of IETS is significantly extended here with respect to both, experiment and theory: STM-IETS spectra of 4MPy/Au(111) are measured at low temperatures and compared to theoretical spectra considering more than 20 transition lines. This complexity leads to completely new insights. Experimentally, the vibrational spectra exhibit a dependence on the specific adsorption site of the molecules. Theoretically, this dependence is only accessible if anharmonic contributions to the interaction potentials are included. These joint experimental and theoretical advances open new perspectives for structure determination of organic adlayers.