Welcome to Sunline Owners Club. Glad you found us and congrats on your new coach.
Now to your light situation. We only have 1 wiring diagram and that is from a 2004 T2499 we acquired when the factory was still in operation. It is here if you want to look at it. Sunline Coach Owner's Club - Sunline Owner's Files - Sunline Solaris T-2499 Wiring Diagram
However we may be able to help with no wiring diagram. I do not know your exact camper and Sunline may have done things a little different in 1989 but here is a place to start.
Are only the lights in the bath room affected? Have you tried all the other lights in the camper? Reason for asking is, is the problem somewhere else also and if so that may help lead you to a problem area.
The 12 vdc to the lights starts at the power convertor and then runs to the appropriate 12 vdc circuit. By turning your lights on that work, you can find the fuse or fuses they are on. Odds are high that the bath 12 vdc is also tied in with some other 12 volt devices along the way. By figuring out what fuse stops the good working lights you might be able to back into what might be the right circuit. The easy thing is no 12 volt power works to other things on the way to the bath. That points to the power not getting to the 1st device or the ground wire for that circuit is not good.
Now if it is just the bath room it could be the hot 12 vdc wire or the ground that is the problem. Most lights are daisy chained together. Both the hot and ground wires go to each light. If the switch is on the light fixture then both wires run to the next fixture and so on. If you take down the fixture you will see the wire and which way they are heading. Then go back to the first set if you can and tighten the wire nuts if that is what they used still back then.
If you have a volt meter you can check each location if the hot wire is hot. Do not use the ground wire next to it for the meter as the ground my be unhooked some how and that is the issue. Find another source of ground. If you have power and no light, then do an ohm check on the ground wire to see if you lost ground.
Are the lights that are not working on a wall switch? if so start at the switch for power and ground check.
This is going to be a search and destroy mission unless you get lucky and find the problem quick. Hopefully you have a volt meter that can read 12 vdc. That really helps cut down on the search.
Once you find the place the power is and then is not then you have to hunt down what let go. Odds are high it is a loose wire nut or a loose ground wire.
Hope this helps get you started at least.
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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