Where is the PCM located on a 2000 Plymouth Neon?

The PCM almost never needs to be changed. Usually its a short in the wiring or even a vacuum leak. The exact wording for a P0622 is "An open or shorted condition detected in the generator field control circuit". A PCM failure is a P0601. You probably have a wire that came loose and fell of somewhere. Dont waste the $300+ bucks for a new PCM till you know thats definitely the problem.