I have a problem with bringing K.A.R.R. back. Here are some of the reasons (sorry this is a long post).
Let's look at K.A.R.R. the las time we saw him in his full form:
The next time we see him after that, here is what he looks like:
And here is K.A.R.R's supposed CPU:

First of all, do we even know that's what it is? Everything I've ever seen or read says that it's his CPU. But a CPU is nothing more than the processor used to run the main motherboard and applications found on the hard drive. So perhaps it's not the main CPU, perhaps it's the AI chip or something else (for the sake of this post though, I'll call it his CPU).
Also, I would think that Charles or someone at the foundation would have what was left of K.A.R.R.'s electronics. I mean after K.A.R.R. was destroyed, the armored car was still on the bridge and would have to be rescued leading to a police investigation as to why the bridge was destroyed and whatnot. I would think that the Foundation would send a "cleaning crew" to the site to clean up anything that was left of K.A.R.R.. Hence the fact that Charles would probably have K,A,R,R,'s CPU. However, we see the flashing CPU at the very end of the episode after we see Michael in a different location and in a change of clothes. It appears to be several days later. So we have two different scenarios here involving the eventual fate of K.A.R.R.. I'll address both and why I don't think either would lend to a good story involving K.A.R.R.'s return.
1. Charles has the CPU
First things first, if FLAG did recover the CPU, I think they would destroy it. There would be no need for them to keep it. What's that? You say they'd want to study it to know why K.A.R.R. went bad? They already know that and it was discussed by Devon, Michael and Bonnie in
Trust Doesn't Rust. And even if they did keep it, neither Charles nor Bonnie would have ever put it into another car. They would have hooked it up to a non networked computer system and reprogrammed it; at which point they would have run extensive tests on the AI to make sure it was no longer a threat. Then they would have deactivated it and still never put it in a car. But of course none of that ever would have happened, because Devon said in
Trust Doesn't Rust that Wilton was supposed to have dismantled the car. Once the CPU was back at FLAG, there's no way he would ever let it be used in another vehicle. I believe he would have had it immediately destroyed without any tests even being run on it. Remember K.A.R.R. turbo boosted into the semi and said
"unless you want to be pressed right into the diesel block, I wouldn't move Mr. Miles." That's not something you easily forget. Devon would have had the CPU destroyed immediately upon its return to the Foundation. He probably would have taken a hammer to it himself.
2. The CPU was never recovered by FLAG and left laying out with the wreckage to be discovered by someone else.
OK, this is the most likely explanation based on what we've seen in
K.I.T.T. VS K.A.R.R.. But for K.A.R.R. to come back as an evil nemesis to the new K.I.T.T., he would have to be found by someone with evil intent who knew what it was that he had. Most likely it would be found by some curiosity seeker who went to see the broken bridge (but the whole place would be cordoned off as the bridge was no longer structurally sound and declared a disaster area) or perhaps a city clean up crew who were cleaning up the area in preparation of the bridge reconstruction (but as the bridge was no longer structurally sound, it would have been demolished soon after the incident possibly destroying everything left of K.A.R.R. by either the dynamite or falling debris) . So some city worker finds the CPU as he's cleaning up the wreckage of the Trans Am. It's flashing and beeping and interesting looking so he pockets it. He shows it to some friends who have no idea what it is and it eventually ends up on a bookshelf at home for a number of years or possibly in the garbage. Cars were being made with more and more electronics back then, so it would probably not look out of place to the average person; especially coming out of a fancy sports car. And if it did end up on a bookshelf, how long would the battery on this CPU last? And who's going to see it and know what it is and what to do with it? Probably no one.
So lets say it's found by someone (who can get by the disaster area barriers) who has a friend who graduated from MIT with a doctorate in computer science and technology. So this doctor friend sees it and takes it into his lab and looks at it. Again we don't know how much of K.A.R.R. is in that supposed CPU. I doubt there's enough memory in that little flashing thing to contain all of K.A.R.R.'s memories and everything. But whatever, this scientist hooks the CPU up to a computer in his lab and finds out that this is some truly advanced form of AI. There would most likely be some way of identifying where the AI came from. Probably just the
"I am the Knight Automated Roving Robot. K.A.R.R. if you prefer" would do it. I believe at this point the scientist would contact Knight Industries and tell him of his discovery. At which point we have Devon with the hammer again. But let's say he doesn't tell Knight Industries. Now he's got this CPU with advanced AI and what does he do? The whole point of the argument to return K.A.R.R. to
Knight Rider is as a supercar villain. I just don't see that happening. I mean even if this MIT computer nerd were to find a brilliant auto mechanic to help him install the CPU into a car, then what? The car would be no different than when K.I.T.T. was a 1957 Chevy in KR2000. No molecular bonded shell, no turbo boost, no microwave jamming, no scanners. How evil would he be? Wow! He could drive himself around, cool. First bank he tried to bust through and he'd have a busted frame and suspension. And someone smart enough to reactivate K.A.R.R.'s CPU would realize after getting to know K.A.R.R. that he was a psychopathic machine; and I don't think he'd install the CPU into an auto.
I guess what I'm trying to say is that to have K.A.R.R. back the way you want him would mean the willing suspension of disbelief within the
Knight Rider universe, where we already willingly suspend disbelief regularly, and that's asking a lot. I know you want K.A.R.R. back for the nostalgia of a favorite nemesis for K.I.T.T. (and it's not even the same K.I.T.T.) but there are too many things necessary to make that happen believably.
Rouge FLAG employee (but I've already stated that there's no way Devon would have let the CPU stay around)
or
Super smart computer genius who somehow by chance happens to end up with the CPU.
Super smart computer genius who knows how to install the CPU into a car.
Super smart computer genius with enough money to get a fast sports car.
Super smart computer genius with connections to Q, James Bond's car outfitter or someone similar to truly make him a supercar. He'd have to find a bad guy Q though.
Super smart computer genius with a doctorate from MIT and everything going for him like a bright future who decides being a bad guy is the better way to go.
Sorry, I just don't see this happening. And unless they can come up with a very plausible way to bring K.A.R.R. back, I don't want to see him brought back just for the sake of bringing him back to please the fanboys.
Again, sorry for the long post.