I have a seat,
http://www.jjproducts.net/ . I rate them first class The prior owner put it in, I emailed for info, they called me !!
It isn't bad, seat back flips up, there are seat belts etc.
You wouldn't want anyone over about 5 riding back there, although it came in handy for a quick trip with 3 the other day.
I'll warn you though, they ain't cheap.
Try and save the original rear deck, I've considered trying to put mine back to original and the prior owner didn't save the parts removed. On my 1990 it replaces the rear deck, I think some later it hooks to it ??
Here's another thread up here about them.
http://www.peachparts.com/shopforum/showthread.php?t=65323