I have the stock seat on my FLSTC and there is enough room for the PCIII. Not sure what you ride. The PCIII was mounted toward the back of the seat above where the battery mounts.
If I had my bike here, I would get you a picture, but here in the Chicago area its been raining on and off for the past two weeks. Sucks
Depends on the bike, on my Screamin Eagle Fatboy, the dealer had to cut a small hole in the bottom housing of the seat, that was the only way, the other option would have been behind the rear finder where it would get too dirty/wet.
my bike is a standard and i had no problem with the install but i just bought a saddleman king seat so i hope
it has the same amount of room to fit as the stock seat.
I wasn't able to fit mine under the passenger pillion of my FLSTC/I as suggested in the instruction. It did fit ok after pushing the fuse/relay block down as shown in the instructions