Again,you need to locate where the power that is supplied to marker light circuit first,once you get that fixed ,the interior light problem may also go away. Don't be afraid to cut suspect wires to see if problem goes away ( they can be easily reattached ) You have a short to power ,somewhere wire that is hot all the time is somehow connected to circuits affected ( someone spliced the wrong wires,smashed harness,even as I said before screw through harness ) take a closer look and open anything that doesn't look factory, another thing you can try is wiggle test harnesses, connect the battery and while someone else is watching the lights start moving harnesses until that someone yells out that they flickered or went out , do this slowly. Don't just concentrate on front of the car , check harnesses under carpets , trunk etc. Good luck.