I wired up the circuit to the switch in the cabin and the relay. The circuit latched on and latched off when the two state momentary switch was pressed on the appropriate side. The internal light illuminated when it was in the on position, the relay also clicked into the closed position so everything seemed fine. I then wired up the heating elements to the battery pack and inserted the relay as the on/off mechanism. I went back into the car to test it. I turned on the switch, the indicator light lit up. I confirmed with my ammeter that the pack had additional 12Amps of draw, so it was on. Then I tried to turn it off. The momentary switch and the indicator lamp turned to the off position, but the meter still read out 12Amps. So something was wrong. I could smell burning under the hood. I pulled the traction pack cable off the relay.
It turns out the relay fused in the ON position the first time I turned it on! I picked this relay because it was supposed to be rated for 370V and 25Amps, but apparently my 250V @ 12Amps killed it on first contact. I opened the relay to see the damage. Here are pictures of the contacts fused together.