Electrical "Ghost"

['76 Big Block]

Went to drive my truck around the block this morning just to warm her up a little---parked and got involved in other things.
Then, went out to get mail, and noticed tail lights/side marker lights were on (they weren't supposed to be)
I disconnected the ground wire, and have ordered a repair manual along with the wiring diagram, but, in the mean time, if anyone has any ideas, I'd welcome them.
The issue didn't appear until I drove it this morning, so I'm trying to figure out what could have shorted or happened to cause this.
Thanks in advance!