While apparently not all that well understood, studies suggest Magnesium may play an important role in Vitamin Di metabolism and perhaps even proper Vitamin D Receptor (VDR) formation. Studies also seem to suggest that magnesium deficiency may lead to Vitamin D resistance:
Interactions between magnesium and vitamin D: possible implications in the immune system
The relationship between magnesium and calciotropic hormones
Bone specific binding sites for 1,25(OH)2D3 in magnesium deficiency
Low serum concentrations of 1,25-dihydroxyvitamin D in human magnesium deficiency