Magnetic precursors of C-type shocks accelerate, compress, and heat molecular ions, modifying the kinematics and the physical conditions of the ion fluid with respect to the neutral one. Electron densities are also expected to be significantly enhanced in shock precursors. In this Letter, we present observations of strongly polar ion and neutral molecules such as SiO, H13CO+, HN13C, and H13CN, which reveal the electron density enhancements associated with the precursor of the young L1448-mm outflow. While in the ambient gas the excitation of the ions and neutrals is explained by collisional excitation by H2 with a single density of ~105 cm-3, H13CO+ shows an overexcitation in the shock precursor component that requires H2 densities a factor of >=10 larger than those derived from the neutral species. This overexcitation in H13CO+ can be explained if we consider an additional excitation by collisions with electrons and an electron density enhancement in the precursor stage by a factor of ~500, i.e., a fractional ionization of 5 × 10-5. These results show that multiline observations can be used to study the evolution of the ion and electron fluids at the first stages of the C-type shock interaction.