97 Mazda B4000 outside mirror question

97 Mazda B4000. Manual outside mirrors. The mirrors will not stay in adjustment. I adjust them, drive around, and a few days later, the mirrors have slipped out of adjustment.

I believe the way to remove the mirrors is to remove the door panels which exposes three bolts holding the mirrors. Once I remove the mirrors in their housings, then what do I do to tighten the mirrors? Or, should I scour the junkyards and replace them, housing and all?

Thanks.