Updated the posted document to include your description of the relay purpose...thanks

+1

A couple more things that work with troubleshooting nearly all rear window defrosters (except some Rolls Royce's that lack defroster grids).

With the engine running and the rear window defroster switch on (and the indicator lit up), with a 12V test light hooked up to a known good ground (I use the door latch striker), slowly, using light pressure, sweep across the grid from one side to the other. Each grid line should light the bulb brightly on one side and as you sweep across, the bulb will become dimmer until going out completely on the ground side. An open grid line will be bright up until the break, then go out on the other side of the break. If the test light doesn't light on either side on any grid lines, check the for voltages at the relay. If the test light is constantly bright and never dims, the ground side is faulty.

There are various repair kits out to repair minor breaks in the grid. The kit comes with a conductive paint and a small template that you place over the break to help you "paint" the thin line.