My back seat use to be gray but has faded/morphed to a light greenish color. Does anyone know if that can be scrubbed back to gray, or any cleaner or chemical that would help?

Sounds like time and the UV rays from the sun have done their dirty work and faded your seat.

 

There must be a product that will "restore" your seat to somewhere near its original color.  I know they have products for dying the vinyl and upholstery in vehicles, don't know why it wouldn't work on your seat.

 

Try googling restore vinyl color, or something like that. 

 

Here's one result.

 

http://www.bing.com/videos/search?q=restore+color+of+faded+vinyl+boat+seats&qpvt=restore+color+of+faded+vinyl+boat+seats&FORM=VDRE