87A is the N/C contact. You actually have a SPDT (single pole double throw relay there. It means that with no power to the coil +ve, current will pass through from the battery to whatever you have connected to 87A, when you turn the key "on" and get power at the +ve, the relay will switch and put power to 87 instead.

In your situation you don't need to connect anything to 87A, just make sure it is insulated from any accidental earth contact as it will have 12V there whenever the key isn't in the "on" position.