Nope. Something so simple, and in a way daft, I dont think I would ever have thought of it but you had the right idea. I thought along the same lines but could not solve it.
I used a piggyback to link into the cars power system. I did it onto a fuse in the box between the door frame and the door itself. Have a read Click me to read
After trying everything I could think of and using up 9 fuses I was told to look into the cigar lighter. I looked. A 5p coin had dropped in there, I had previously removed the lighter itself, and was resting in the bottom. The coin was shorting out and blowing the fuse. Who would think of looking in there!