We study Anderson localization in disordered helical conductors that are obtained from one-dimensional conductors with spin-orbit interaction and a magnetic field, or from equivalent systems. We call such conductors ``quasi-helical'' because the spins of the counterpropagating modes are not perfectly antiparallel and have a small spin-wavefunction overlap that is tunable by the magnetic field. Due to the overlap, disorder backscattering is possible and allows a localization transition. A conductor can pass through two localization transitions with increasing field, one from the conventionally localized system to the quasi-helical conductor, and one at a higher field again to a localized state, due now, however, to backscattering below the magnetic field induced pseudo-gap. We investigate these transitions using a unified 2-step renormalization group approach.