I would think the problem is that it will measure continuity when cold but maybe some thing opens up under load. useas volt meter or test light. Clip one lead to the power lead on one side of the window. Then run other test probe along each trace from side to side. You should read zero volts near one side then up to battery voltage on the far side with no sudden jumps. If this is a wagon suspect the wires that bend when the tailgate is opened. For sedan and wagon check howthe wire is attachd to the glass. These fitting oftem work lose over time and do make good electrical contact.

Bill D.

ps be careful with the probe not to scratch through the baked on traces on the glass. Any chance there is a foil decal on the traces to short them out?