LR window works at rear door switch but not at driver's switch

My left rear window goes up down fine when the left rear door switch is used. However, it does not move at all when the driver's door switch is used.

This seems to point to a faulty switch at the driver's door or wiring to that switch. Any other possible diagnosis that I'm missing?

Can you purchase just one switch or do they come pre-assembled in the panel of four?
Anyone have any DIY tips?

Thanks!