If there is only one vent, then someone put a wall up and cut a larger room in half. You should check and see what having another duct installed would cost. You can have a T put on an existing duct to vent to the other room. It's a jumper duct. It will also help with the AC. If the space heater is tripping the fuse, there is too much pulling on it already. Each circuit can only handle X amt of electrical pull before tripping the breaker. You can try turning everything plugged into that room off and see if it trips again. It's a fire hazard for the most part.
I just don't use it. I always thought that there was some renovation done. The layout of this house has me puzzled. It's weird but I like it. Thank you.
You can always insulate that room without the vent. You can buy insulation boards that attach to the sheetrock. Turn it into a utility closet or something. Put a door draft stopper at the bottom of the door that connects the two. The objective is to keep the cold from the non-vented room from seeping into the others.
It's a bedroom/playroom and I just keep the door closed since it is not in use. Good suggestions I appreciate them Rene. Keep them coming.