At least you found the source of the leak. Take the good with the bad. Now, about fixing it.
You have a couple of methods. Here is one I like. Get a heat gun. They range from $30 to $100. You only need the $30 Walmart special for this.
Get a good putty knive with about a 1.5" wide blade. Keep a hose or extingisher handy in the event sparks fly. Although, they shouldn't unless you arch off some old pine needles or leaves.
You can use the heat gun to heat the area and push that tar right off and out of the way. When you have cleaned enough of it back, you can use laquer thinner, acetone, or gasoline to finish cleaning it up. Let the cleaning solvent dry on out for an hour or so. Now you can rough the area up and lay down some figerglass cloth/mat in the area. When it dries, sand off any high rough spots, apply as many layers as needed, then get a good petroleum/tar based undercoating spray (rubberized if you can't find the tar base) and respray the wiper channel. Good as new!
:cool