electrical issues (horn, airbag, headlights)

Hi everyone! Long time lurker first time posting here. My 2000 prerunner project truck came with some strange wiring issues and is basically a death trap in its current state. The truck only has 140k miles and runs great but needs some tlc.

Issues:
1: horn is not connected to steering wheel (aftermarket horn button installed by previous owner)
2: burning plastic smell when headlights are on for more than 5 minutes (aftermarket headlights)
3: airbag connections are spliced with crimp connectors
4: mysterious black wire hanging from steering column

My new replacement steering wheel from a 99 tacoma has a plug connector for the airbag wiring. Can I cut the spliced wires on the old wheel and add a connector to make the new wheel plug and play? I have read in other threads that even if the battery is disconnected, the airbag can still go off.

Could the burning smell be a bad headlight switch or a bad ground somewhere in the circuit?

What could this black wire be?

Any help is appreciated, I don't want to spend thousands on repairs but I will probably end up going to a shop for all of this.






new steering wheel with connector