2002 Dodge Ram Lamps Out

Driving home yesterday after buying a truck the Lamps Out waringin light came on and continued beeping the last 50 Milles home. Once I got home I found all running lights out and drivers side head light.
Brake lights work, bright lights work, turn signal lights work each indepentedntly but when I turn on the light switch nothin works but the passenger side head light. The error code I think was PO300 I Tried to confitm and it just tells me "done" I check all fuses
and swaped 2 50 amp relays. What is casuing this, I cannot drive this truck in this condition. Thank you in advance for yourt time

Jim