The rear seat is screwed to the rear firewall from the engine side. The rear firewall must be removed from the Pulse to get at the screws.
First remove both side panels. Then remove the screws holding the rear firewall in place. There are 4 near the top and 2 more down near the floor. Then the rear firewall will be loose. It will take a bit of jockeying it around to get it out of the Pulse. It's a tight fit. Then unscrew the seat pads from the firewall. Because the rear firewall fits so closely to the frame behind the seat, the original rear seat is the best fit.
Finding a Replacement Seat
Here's a couple of good seat sites:
Cabelas web site......