[ImperialCrown]

Welcome to Allpar. The mechanical high beam detent is inside the multifunction switch. If it has lost its 'click' and latching, then the problem is likely internal to the multifunction switch.
The 'no bus' message will need to be diagnosed and may the reason why the car won't shutdown and your instrument cluster remains active after key-off. The PCI communications bus is a single Yl/Vt wire that moves information (or not) between the various modules. A little wetness, corrosion or electrical leakage at a connector can take the whole bus down. It can be pulled high or low. A DRB III scan tool is easiest for getting fault codes from the BCM, cluster and PCM and running the various diagnostic tests.
If you want to pursue finding the bus fault yourself, you will need to start with a voltmeter, the factory service wiring diagrams and body diagnostic procedures books:
http://www.amazon.com/Chrysler-Concorde-Intrepid-Service-81-270-8140/dp/B001DU7KOA
http://www.amazon.com/Chrysler-Concorde-Intrepid-Diagnostic-Procedures/dp/B002TFZYLY

 

[ImperialCrown]

I would do some diagnosis before replacing anything. Your old BCM probably was not the problem and the PDC may not be as well. I suspect wiring and connector corrosion or leakage for the bus communication issue more than a component that has failed.
The clicking relay can be found by feeling which relay is clicking. It could be a poor connection or poor ground or an electrical feedback loop situation which usually comes back to a poor connection and probably not the relay itself.
Your normal diagnostic tool is probably only for engine fault codes and won't help very much with body electrical. You still would be better served by a factory service manual for both diagnosis and procedures.
If you trust this website and can open .rar files, then downloading this service manual may help. 1998-2004 Intrepid, Concorde, 300m and LHS were all similar cars. Otherwise a public library search or service manual purchase may be the best way to start.

CHRYSLER-300M-CONCORDE-AND-INTREPID-2004---Service-Manual.rar