I've used that add-on and it's really useful but it won't help in this case unless a person does manual searches for each batch of threads they want to update. To clarify that, if you wanted to automatically update all threads that have the word "ford" in the the thread to also have "ford" as a tag then, yes, you could use the add-on to search for all threads with "ford", update the tag, but then you'd have to do the same thing over & over again for "chevy", "gmc", "jeep" and so on. The OP is after a way of automating the process.
I have been looking for this as well. As @Kevin pointed out i have used the method described in his post but its extremely tedious. Would be great to have a blacklist of words in the title not to autotag and perform autotag on the rest.