Rubber Repair
If the tear is not too, too bad (actually you can bridge any length), get a piece of small soft tubing and bridge the tear. Go into the hollow of the weatherstrip with the tubing.
Buy some black silicone rubber. I buy it in the caulking gun size.
Overcoat the weatherstrip with the silicone rubber.
Now here is my very own patented method for repair.
Get a piece of plastic wrap and cover the repaired area completely.
Smooth the repair thru the plastic wrap. Wet your finger and it will slide easier.
Close the door. The door will make the final shape/seal. The plastic wrap prevents the silicone from sticking to the door.
You can use the car normally for the next couple of weeks, or forever for that matter.
After a few weeks, the silcone should cure. If not, leave the plastic wrap on for as long as it takes.
Now you all know what I know, and I will send anyone who knows why this method works a check for $100 (but I charge $100 if you use my method).
Here's hint, don't even try to use Saran Wrap.