For clear plastic, I wouldn't use any sealant. For all other plastic, I would use the Wolfgang Cockpit Trim Sealant. You can use it on your leather too.
You only stated what material your seats were made of and not how dirty they are, so I will give you a wide explanation to how I clean leather seats. I start with the least aggressive method, if that does not work, I take it up a notch untill I see my desired results.
Starting out, leather cleaner is safe, of course. If that didn't work, I would try dilluted alll purpose cleaner. I have even needed to use dilluted heavy degreaser before. Scrubbing with a softer brush after you spray the coated leather works wonders. Often, when you lift dirt out of coated leather, it comes out a little at a time, so miltiple applications probably will be required. On lighter colors when the dirt lifts, you may see some lightening of the color and confuse this with lightly bleaching the surface.
Just ensure even coverage and thorough effort and you'll be fine. If you use ant thing other than a leather cleaner, perform a final wipe-down with water and dry before applying a sealant.