Seems simple but it's the simple things that get overlooked
Has the battery been tested? 12volts? Are the connections clean and not corroded? Are the wires going into the connections clean and not corroded/twisted? Are all fuses good(take them out and test with multimeter) Do you have a good ground- all grounding straps clean and making good contact? Does your rig have a "salesman switch" above the door, next to the door on the cabinet as you come in?
Relays and Salesman Switch Questions
If so do you have power on both sides of the switch? Again multimeter or test light required to check. Let us know what you find