I'm not entirely sure what/how the system you're installing controls, but I don't think there's a "heater relay" that replicates the function needed.

In your install, does the system energize the car's heater system, or just the blower on low speed?

The drawing on page 9 of your link looks like it recommends splicing in a changeover relay between the low-speed control (which is behind the glove box, I think) and the resistors. I think the resistors are on the passenger fender in the engine bay. That changeover relay would would use "car" power for low speed blower when the espar thermostat is not engaged, but use "espar" power when the thermostat is engaged.

I think the "heater relay and fuse" mentioned on page 9 to the right of the changeover relay refers to the espar relay and fuse contained in your kit, NOT one already in the car. If you look at the illustration and instructions on page 12 it points the black wire back to the espar box. This would seem to make sense with powering the car's blower and the thermostat via the espar unit and not via the car's wiring.

The relays under the black box on the DS fender are the A/C compressor relay and the aux fan relay. There may be others there, but I can't recall of the top of my head.