Junction block
 

I have a chrysler 300m 2002 and the air, windows, radio,turn signals, and sunroof won't work. I took it to dealer and they said probable junction block and maybe bcm. I heard that the junction blocks rarely go bad. Can I purchase a junction block from a junkyard and is it hard to install? The price from the dealer was 2000. and that is too high.

A big part of restoring is working without factory parts.
A horn relay is just a relay.
You could easily wire in a standard Bosch relay and use the horn to ground the relay contacts.