12 volt light fixture problem

I left a 2 bulb wall fixture on when I left the rig. When I returned, the fixture was off. I checked the bulbs in other fixtures, and both were burnt out. I attempted to replaced with new bulbs. With the push button switch still on, when I would insert the new bulb in either socket, the remaining lights on the circuit would fade to off. I removed the fixture from the wall, removed the switch and checked it and the fixture for continuity. Both were okay. I checked voltage on the line and it is okay. The fuse was not blown. All other light fixtures on the circuit work correctly. So, my question is, fixture or wiring or something else?