Can I tile over my painted bench Seat?

I don't see why you couldnt. I'd suggest scoring the surface that you are covering. I personally would use a circular saw having it go about 1/8" into the surface so that the mastec, had something to grip onto. Or you could just try using liquid nails. That way you know it will stick.

What is your bench seat made from? It is completely flat? Could you add a section of plywood and tile that instead of directly into your bench seat? The reason I say that is because if you tile and grout over your bench seat removing things later could become difficult and would basically destroy the top of your bench seat. You could use VINYL tile flooring--comes in squares or rectangles--lots of colors and designs. Removing that at a little date would be much easier and much less destructive and extremely reasonable and durable. OR--another idea is to add wood to the top of your bench seat--similar to the look of the top of a farmhouse table or bench--2x4's or 1x4's or 1x6's or 1x8's--stained and sealed with poly.

You can tile just about any flat surface. You can grout it or glue it. I have done table tops on plywood it worked out just fine. Just remember if you grout it on, you are pretty much stuck with that.

Use contractor's glue to put down the tile rather than mastic and use a tile that produces a very thin grout line, such as granite which can be placed next to each other with a hair line grout line. This will prevent the top of the bench from warping. Good luck!