Recommended prep prior to installing new door locks/latches?

Hello - I have a 1985 Defender 90 Station Wagon, and just ordered new door locks for both my driver and passenger door, so this way, there will be one key, to open all three doors.

Now - I previously installed the rear door lock/latch and the problem I come up with is that sometimes it works, sometimes it does not. If it doesn't, i have to in from the inside of the Defender, and push the little latch to the unlocked position, to get it to open. It's being a bit of a nuisance at this point.

I really don't want that to happen, once I swap out the driver and passenger side door locks. (Don't feel like having to break a window to get into the truck.)

With the rear door lock, I was first shooting it with WD40 to get it to open at times, but I know that would cause problems later on. From there - I started using a graphite key lubricant, and it works a little better, but there are still times where I have to unlock it from the inside.

Has anyone else had this problem, with doors not unlocking? What efficient prep should I do for the new ones, and for the rear door, should I take it apart, clean it, then reinstall - and reapplying the graphite key lubricant?

Thanks in advance!

I have swapped out two of these into the rear door of a buddy's truck and both have failed. I think I am going to disassemble it and weld where the lever is just crippled onto the main shaft of the mechanism.