Do these connect to the factory harness, or do I need new wiring?

These mount directly to OEM wiring points—no additional harness or controller needed. Plug them in at the factory connector and they're live.

Can I use just one mirror, or do they have to be replaced as a pair?

You can replace one side at a time. Order left or right individually depending on which mirror you're replacing.

What's the return window if the mirror doesn't work after installation?

Confirm the return policy directly with ModSupply before purchase—mirror returns are typically processed within a set window after receipt, but specifics depend on condition and installation status.