Door Will Not Lock - 2013 Volkswagen Passat TDI

My Passat has a locking issue with the driver's door. The door switch works (actuates other door locks) but the driver's door wont actually lock. I had an extra door switch so I swapped them just in case that was the issue, but there was no change.

I've been tinkering with electronic projects for over 20 years but I never had the opportunity to learn proper automotive electrical diagnostics. I've never used a test light, just a multi-meter. However, I had a chance to get an Icon digital test light/circuit tester (in picture) for a steal and I couldn't pass it up. I'm hoping you guys could walk me through this diag, and maybe put the new test light to use. How would you pros go about this? TIA!

Congrats on the test light! 

Unfortunately, your testing options are limited without removing the drivers door panel. The drivers door lock motor is run by the drivers door module, both beneath the door panel. If you had a full function scan tool, you could query the DDM for fault codes. 

The only non-invasive test option you have is to find fuse #31 (30A) in Fuse Panel C and measure current across the fuse while hitting the door lock button. If you saw an increase of several amps across the fuse after hitting the lock button, I'd be confident in a binding actuator and go for replacement. If you saw no current change, you're back to pulling the panel. 

With the panel removed, find pin #12 violet and #13 yellow. Those go to the door lock motor. Hit the lock button while probing each wire with your test light connected to B-. Then probe each wire connected to B+. One wire should light on B-, the other on B+. You're checking for power and ground coming out of the DDM. If they're there, it's actuator time. If one or both is missing, stop and tell us so we can go down another path.