Phone won’t vibrate after replacing Taptic Motor

It's something to try, but why would it work with the original and work with the replacement if the phone was booted up with the original but then replaced after the phone booted up?

Could be a dodgy replacement part, i would really encourage you not to hot swap components (swapping while powered on). If it wont boot with the replacement part, Theres a good chance the part is at fault and being they are so cheap i would just grab another one at the same time you grab a flex. This should solve your problem.

Yes, and try to get your hands on a known-good part so that you can eliminate some variables. If known-good parts don't work, then the issue is probably with the Arc Driver circuit on the logic board.

Okay, that makes sense. I bought the Taptic Motor from iFixit, so I'll see if they'll swap it out for another one (unless you have a place that you recommend getting the part from?)

Now, if I replace the motor again with another new part, and replace the charge flex and it's still not fixed, Minho stated the issue is probably the Arc Driver circuit on the logic board. I'm not all that familiar with iPhone parts like that, just the basics. If it's the Arc Driver circuit, is that something that is fixable?

Logic board issues would have to be looked at by someone who does micro-soldering. Focus on the modular parts for now.