If you do fit the heater elements they're combined power use is less than a bulb so no relay is needed if you get a 15amp switch and basically you can just take a fused ignition live wire through the switch to the heater elements so long as you remember not to leave them on for more than 15-20 minutes at the most or you risk burning them out. I have had them fitted ages with no issues as I used a switch with a light.