My brother-in law has been having problems with his truck. He replaced the battery and alternator. Occasionally it will start, but usually when he turns the key it just clicks, then all the lights go dead. If he leaves it alone everything will eventually come back on, but will do the same thing if he tries to start it again. Any ideas??

Most likely a bad starter or horribly bad electrical connections to the stater, pull the starter and have it tested if the wiring checks out fine.