Simple fuse box fix?

I trouble shot a 1968 Hemi Roadrunner once that had similar problems. Everything was brand new. Battery fully juiced. Dome lights came on when the door was opened but when you tried to start it, everything died. I tracked it down to a new positive battery cable that had a factory defect. The crimped lug on the end to the solenoid wasn't crimped enough to carry the starting current. Replaced it with another new cable and the problem was permanently solved.

But don't just start replacing things to see if your problem is similar. Follow the advice you are being given in this thread to track down your specific problem. These guys know what they're doing! Plus, you will be proud that you fixed it yourself (with advice) and you will have learned valuable skills in the process!