limp mode when placing transmission in reverse with lights on

I recently bought a 2009 Expedition XLT with 130,000 miles. Someone had cut the cats off of it so I just bypassed it with straight pipe. I don't care if lights are on the dash or not. It typically is a great running machine. What has been giving me problems is that when the lights are on and the vehicle is placed in reverse, everything on the dash starts flashing and different warning lights start cycling and then the wrench comes on the screen and the vehicle is in limp mode. If I turn off the key and start it again and place the transmission in a forward gear it runs perfectly, until it is placed in reverse again with the lights on.
I'm new to Fords so I was wondering if this sounded common to anyone? Is it a wiring issue? Transmission issue? Like I said, only when the lights are on and the vehicle is placed in reverse.
Thank you for any help.

Hook up a scanner and see what codes are being thrown. Also check all of your ground wires. A bad ground can cause some wonky issues.

That's my guess is a harness was damage during the cat removal, or maybe it is sitting on the new straight pipes and melted.

Thank you for the responses. Hopefully I'll have some time in the next week or 2 to check it out.