there are different problems with the pre (the ones with the seperate door opening handles and little red buttons to open/close the locks) and post 90 spec cars (handle push pull to lock/open)
the earlier cars used rods to operate the locks, these were easily greased on the little plastic guides once the door cards are out of the way and should solve the problem
the later cars are much more complicated (and operate the opposite way round too) and are much better sheilded and more difficult to diagnose
the first things first, remove both front door cards as both the front motors are masters to trigger the rest of the system, these have 4 wires on the plug while the rears have only 3
once you have the door cards off find the motor connection and unplug one side, then operate the locks to see if the problem is solved
you need to check the locking at each stage of dismantling
do check the other side too because it can affect both at the same time
once you have found the correct door start by stripping the lock out, this is seperate on the pre and fully inclusive in the post 90 spec cars
if you have the pre cars there is not much more to do
post cars then need to have the metal window slider removed (some pre cars have a metal cover to protect the locks too but not all of them, they need to go back in and there is a big screw at the bottom of the door that holds the whole thing in plus a clip into the window runner channel)
now you can remove the lock mechanism
there should be a plastic cover on it, 2 plastic pins hold it in place, pop them off and carefully start to dismantle things until you can check the workings
if it's not the motor it'll be the lock internally, they can break and then you have to find a replacement
they are sided too
be very careful when you take stuff apart and don't lose anything because the little bars and clips are specific to each lock, and colour coded too in the plastic parts
if this had been 10 years ago i would have been able to stick pictures of everything you need to check
