Many of the vinyl-plastic comp[ounds discolor with age and not much can be done. Velvac can provide a new mirrow housing though, and their parts prices aren't unreasonable. They will sell direct to you via their web site.
I did have moderate success restoring a refrigerator outside grill cover once. It had yellowed badly and it was fix or replace, so I tried everything. The only thing that worked was lacquer thinner, which actually softened and removed a bit of the surface. If I wiped it across the surface I got a decent white color, but it was hard to get it smooth and unstreaked. Took me awhile to develop my technique, but I eventually got it more-or-less acceptable looking. You could try it IF you are prepared to buy new housings anyway, but I'm not promising acceptable results. "Your mileage may vary".
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL