FWIW - the BMW Parts Book has...…….
51 31 1 350 309 - rear heated window kit, comprising switch, cables, rear heated window & panel for ? underdash.
51 31 1 356 372 - rear heated window kit, as above but no panel. This one is also called up for the 1502; guess that model was a real basement bargain.
Someone better at deciphering the parts book than me may be able to see what EXACT years/models the above applied to.