Trailer wire harness install problem

I purchased a Tekonsha 118610 trailer wire harness and was installing it on our 2016 Prius v tonight. I first removed the access panels to access the tail light wire harnesses on drivers and passenger sides, and clipped in the T-One harness right and left without issue. The harness runs to a T-connector box, a potted credit card-sized piece of plastic from whence the trailer wire harness, a positive (red) lead with 10amp fuse holder, and a negative (white) lead depart. After connecting the tail light harnesses, I went to connect the white wire from the T-connector box to the ground screw on the vehicle chassis by the battery and was surprised to get a large spark. The car has now appeared to lose all power - the interior lights are off even with the hatch open, and the power button does not light up.

I watched the install video at e-trailer, and followed the instructions in the kit. The 12V battery is still connected; I did not touch it before or after (I have not connected the T-connector to the positive terminal yet). What have I done? What do I do now?

 

I have obviously blown a fuse somewhere, but before I get to that, I need to figure out why reconnecting the ground terminal to the chassis causes sparking. I'm getting 12V on my multimeter from the negative terminal coming off the battery to the female threads on the inside of the body panel where the grounding screw would go (touching the outer chassis sheet metal reads zero; it's fine). Does anyone have any ideas where the power leak to the threads is coming from? I had no trouble unscrewing the assembly. This should be straightforward but I am baffled.

 

Thanks for the insight. I can check for this, but I'm not sure why attempting to reconnect the negative 12V battery grounding lead to the proximate chassis ground screw would still cause sparking if the fusible link in the positive battery lead was blown.

 

Thanks for the explanation, very much appreciated, and yes that makes sense that the coated/anodized shell is not conducting current, only the threads. So, how would you suggest I proceed? Currently, 12V battery leads are connected, but negative lead to chassis is not for fear of more sparking. All car lights and power are out - car looks dead.

 

Just to be sure, I took a piece of wire and connected from the chassis ground to a 40W lightbulb to the negative battery terminal. Not even a faint glow, so indeed just voltage potential of an open circuit as stated. So, I disconnected the positive battery lead, screwed the ground terminal back to the chassis (with trailer ground terminal) and then reconnected the positive lead. Systems came back up, took the car around the block, and the trailer lights even work.

Thank you rjparker for the helpful explanation and diagrams. I have working knowledge of electricity but sparks flying when grounding at the chassis was unexpected. (I also half expected some reader might give me a lecture about the dangers of towing a 5th wheel with a Prius v). My little utility trailer and I are back in business. Appreciate it, and I hope this thread helps someone else.