Great idea this, thanks Richard. That's a nice small relay. Perfect for the job. I was going to mention that someone could benefit from a schematic showing the coil and contacts, but that's shown in the third photo down. Responding to Bill's question, the relay needs what they call a C-FORM contact set, with one contact normally open and the other normally closed. If you need to find one other than the part Richard mentions, ask for "SPDT" (single pole, double throw) 6 VDC (or 12 VDC, if that's what your running) and 30A contacts.
Raindodger's point about being dipped when released (normally closed) is good advice because relay coil or switch failure results in staying dipped rather than bright until repaired.
Richard L.