If I have a tear or rip in the fondant on my cake, How do I blend or seam the crack?

Maybe try RI in the same color as the fondant if it's just a small tear?

Gently push the rip back together. Use a bit of shortening on your finger to smooth the edges out. If there is still a mark you can fill with royal or buttercream icing and smooth with the heel of your hand. If the area is shiny you can use a bit of PS to rub the shine away. (if you fondant is white)

And if you can, cover it with a design element...but then I'm sure you probably already considered that! Good luck, it can be done...or hopefully be the back of the cake.