For the heater to be at fault, the "cold spot" would have to be a pretty large area. There are multiple circuit paths snaking back and forth on the heat bed - I think it was four. They go from left to right, so if a trace opens up, that entire 1/4 of the heat bed wouldn't be heated. ie, the back 1/4, etc. Shorting out an area of the board would leave unheated areas, but tougher to do. You have foil under the glass, right? Is the heat bed mounted logo down? That would put the heater traces up against the foil and perhaps allow the foil to rub through the covering on the board and short an area. If the heat bed is mounted logo up, look underneath where your binder clips go. Have any worn through the board covering so that traces are exposed? That could short out an area, but this would still be across the entire width of the heat bed. Now thinking of that, adding a layer of kapton along the heat bed edges for protection might not be a bad idea.

I remounted my extruder fan so that heated air exhausts the front of the printer. That's kind of annoying if you're down close trying to look at the print, but I read elsewhere this did a better job of cooling the hexagon hot end with bottom of the X carriage being so boxed in.