Switch was fine, no melting at all. The wiring to the switch didn't look to have any damage either but I didn't dig deeper into the harness. Put everything back together and here's the symptoms currently:
Lights work fine with the trailer light relay out.
With trailer light relay in, turn on lights and once switched off, the parking lights stay on.
Pull the relay and they go off, put it back in and they stay off until the lights are turned on again. Switched relays and same symptoms.
Same symptoms with or without trailer connected.
Turn signal/ brake light/ABS light issue hasn't reoccurred.
Any other suggestions?