Here's a suggestion, and while I have not really looked at the actuators in the 4 runner's doors, these are pretty simple units for the most part. Its literally a motor with some gears that force a plunger in and out. If the plunger is not working at all, the motor has either burnt out or it has a break in the wire. Whether the break is internal windings on that motor or somewhere else inside the unit (I assume the break is inside the unit since replacing just the unit fixes the problem), a few things could be done to repair the existing unit.
Pull the unit out and take it apart, see where the issue is. You could also try to locate just the motor itself, as I'm positive its being used in lots of other products, including electric mirrors, shavers, fans, etc. Places to look for these motors would be Digikey or Mouser, as well as Grainger and other online stores that carry electric motors.
Now, if the motor still turns and you can hear it running when you try to lock/unlock the door, then its the gears that are most likely stripped, and that tells me the motor is either too strong for the locking mechanism, the unit is pulsing way too long, or the throw on the actuator is too long. While harder to remedy, the only other things would be to fabricate (ie., jury rig!) some sort of rubber bushing where the actuator attaches to the door to give it some cushion to help from stripping again.
Of course, there is yet one other way to remedy this issue, especially if you just don't wanna screw with it at all! Head to your local higher-end audio/alarm shop, tell 'em you're tired of burning out your door lock actuators, and you want them to install a "high torque" actuator (I like the Crimestopper brand, that thing is STRONG!!!) in your doors! Its only two wires that control this unit, and unless Toyota decided to add the relays into the actuators, there should be just a 2-wire plug going into it.
And, if you're mechanically inclined and have time to spare, just pick the actuators up online, then determine the best way to mount it in your door. The unit comes with a rod and coupling hardware, although you will probably need to use some of the existing stuff in your door to make it functional. This is where McGuyvering stuff comes in handy!!! :flame:
