I always use a soldering iron to repair cracks. One point to add is that if you do need to weld new plastic in make sure the two types of plastic are the same. The plastic can usually be identified by a code on the back. It will be something like "ABS", etc.

If you can't find the code just make sure you use a piece from that same source - ie a piece of bumper to repair a bumper , a piece of side molding to repair a side molding etc etc .