I would check the headlight switch and dimmer switch. It's best to have a wiring diagram to test these so you know which wires to check. You can get a good wiring diagram here: http://bit.ly/mitchell_repair_manuals_online You can use a test light but a voltmeter is preferable. Check the power wires that go from the headlight switch to the interior lights. Good Luck!
As far as interior lights,my '91 Sonoma has rubber grommets around the door switches that have deteriated,letting sand,dust,etc. get into switch.Just spray some lubricant in there & push in/out & see if dome light works.