I agree with elgordo. I have seen a few bad starter kill relays go bad over time at my shop. The pop you hear may be a bad relay under the dash. Worst case senario just bypass the relay itself. Resolder the starter wire back together to try and trouble shoot one less possible problem out of the equation. Relays by nature are only 30 or 40 Amps. I doubt the car being in gear would draw that much current, but I've seen some weird things before. That would be my first step.