I wouldn't even bother with it. Lifelike is of poor quality. The best way to do this, if you had to, is to remove the gears. I don't know how easy it would bee to open up the truck, but simply removing the motor will not do the trick. you need to at least get rid of the idler gears so that the axles are spinning freely. I have no idea how you'd take apart one of those trucks.
As for the passenger car, there is no need to fill it up with tape and hot glue. This is counter productive, and will be a real problem later if you need to take something apart. Glue can also screw with the weight and running characteristics, and tape doesn't hold forever.
It seems like you stripped the threads where the screws belong to. This is usually caused by over-tightening the screws that hold the body in (to a lesser extent, being yanked or forced). It is important to avoid tampering with these screws as much as possible. Plastic is soft and constant removal and insertion of the screw will eventually destroy the threads, even if you do not over tighten them.
There are two ways to fix this. Buy a slightly larger (maybe only the next size up) self-tapping screw, and screw that one in. Walthers sells these. It will resist at first, but once you screw it in all the way, you should be able to remove and intall the screw easily. Again, you should avoid doing this to much, only when necessary (such as lighting the car or adding weights).
the other option is to find a tube that is the about the right size for the screw, and drill a hole into your model large enough to accept the tube. You can then super glue and sand smooth the tube to wherever the new threads are needed (keeping the glue outside of the tube). A more resilient way to do this would be with a brass tube that is tapped for the screw you use, though this is probably not necessary.
Trust me, I've learned the hard way, i have every sort of amfleet you can get, a bunch of the Walthers horizon and viewliner cars, and they all have the same problem, especially the "used" ones, and ESPECIALLY around the coupler box covers.
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Elite Juice Jack Modeler.