Will this fit if my truck already has a power mirror on the right side?

Yes. This is a standalone left-side manual mirror and operates independently. You can run manual on the left and power on the right without any conflict.

What fasteners do I need to install this?

The mounting hardware to secure the mirror to the door is not included. Check your truck's service manual or existing fastener specs before installation—most stock bolts will work, but confirm your particular truck's door bracket setup first.

Does this mirror come with the glass already installed?

Mirror assembly ships complete and ready to mount. Glass and adjustment mechanics are pre-assembled, so you're installing the whole unit as-is.