Weak Approximation on the norm one torus

For any abelian group A, we prove an asymptotic formula for the number of A-extensions K/Q of bounded discriminant such that the associated norm one torus R^1_{K/Q} G_m satisfes weak approximation. We are also able to produce new results on the Hasse norm principle and to provide new explicit values for the leading constant in some instances of Malle's conjecture.