On three separate occasions, after installing a child’s car seat in the middle driver’s side seat, the battery has gone dead. The seat heater was off. Are electronics in the seat itself, or extended seatbelt draining the battery?
It should not be too difficult to find with a meter and a little effort for someone with a little electrical background. I doubt if this is going to be easy to identify on-line, but maybe someone has had a like problem and can help you out with more specific advice.
I would suspect the seatbelt being clipped in may be causing the trouble but it seems to me that the power should be turned off with the ignition switch so it shouldn’t be a problem. The trouble could be easily checked out by checking the current from the battery as JEM suggested.