Hi there,
Physically moving the passenger front door lock with my thumb works fine to lock and unlock, as well as using the key in the door. The power lock used to work about 80% of the time but it has stopped being able to click the lock mechanism enough to engage/disengage it at all anymore, but it still tries. What part might I try to replace first?

 

Before trying to replace parts, I would open it up and look at the mechanism.
It might just need to be cleaned up and re-greased??
Make sure the linkages move freely.
If it still doesn't work then it would most likely to be the door lock actuators.

 

My advise is:don't worry about that,lock it manually.Avoid headache and tons of labour,that may not be always successful.