I actually test fitted mine with the OEM seat as my Corbin was at the upholstery shop at the time. I did end up squeezing almost all the RCA connectors back into the box because there was not enough room for the seat otherwise like you said. But I got it to fit.
Yes the resistors come with the PAC unit. I think there should be a link to the online resistance calculator that is helpful in determining which resistor combinations work best. When you get it I can do my best to help you from this side of things.