The only permanent fix is to replace.
To save the foam, however, you can patch it from the inside.
It is as much or more work than replacing the cover, as to patch from the inside it must be removed.
Once removed, find a piece of cloth reinforced vinyl, and cut it four inches longer than the split length and 4 inches wide. The idea is to have a 2 inch overlap around the entire tear and you will glue the cloth side to the cloth inner side.
Get a product called Shoe Goo (hobby stores, shoe repair). It is as flexible as vinyl and quite tenacious on clean surfaces. Then arrange the seat cover on your bench or buck, so the repair area is flat and you can fit a caul and weight over the area to press the patch onto the tear.
To hold the seam together before the gluing process, you could use painters blue tape if there will be little stress while clamped, and carpet tape if there will be higher stress during the clamping. Either of these tapes will be applied temporarily on the outside during the repair process. Clear packing tape might let you see the seams appearance before gluing, but I didn't try that.
When everything is all set up for clamping. Apply the Shoe-Goo to the patch and put it in place as quickly as possible, then clamp.
This method worked so well for me that it never split at that point again. A week later it split on the other side. I repaired that in the same way. A week after that it split down the middle of the seat. Repaired. Another week a new split slightly off center to the left. Repaired. Then another one slightly to the right. This process repeated until I had patches overlapping patches. When, the weather got colder the splits began to work their way towards the back of the seat. None of the patches ever came loose.
I bought a replacement cover and haven't patched anymore since. I did save the foam, though.
Best of luck!
