In this paper we calculate the complex dielectric constant of polar liquids for microwave and far-infrared range of frequencies. We use the formalism of extended irreversible thermodynamics (EIT) to obtain the autocorrelation function of the polarization vector. To this end, we introduce the local variation of the polarization flow as an independent variable, which allows for the description of collective interacting effects into the formalism. The results obtained here are compared with previous work in tile so-called three-variable theory based on the Mori-Zwanzig projection operator technique. This comparison provides an interpretation of EIT in terms of a microscopic theory from which the role of the new independent variable can be established. Using the result of the complex dielectric constant presented here, we show that the fitting of experimental data in polar fluids can be improved in the far infrared frequencies.