actually there are 3 12V inputs, one is constant 12V, one is 12V when the key is turned and one is 12V when the dash lights are on. I am not sure which one carries the bulk of the power load but if you wired them backwards it would probably blow a fuse. The only strange thing is that if you weren't getting 12V constant i don't think the radio would work at all, and I know for a fact it wouldn't work if your accessory wire wasn't hooked up. I know on my car the the accessory wire for the clock and radio are wired together and fused in the box under the hood, while the 12V constant is fused under the dash.