If you're lucky just the rear bulb in the domelight may be blown. In any case, you probably need to pull the dome light assembly to check what's going on visually and/or with a multimeter. The dome lights in the 700/900's are poorly designed. The plastic gets brittle and the mounting legs easily break. To remove, pull firmly straight down on the assembly only at the front without wiggling or twisting. Don't push on the front lenses, they push in and break off easily. Additionally, the plastic mounts for the front switches can break if the switches are pushed in too hard and the switches will simply fall apart. If it turns out the timer section for your rear dome lights has gone you can easily bypass it. The units are expensive to replace with new ones (about U$50) and finding a good used one is often difficult.
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Dave -not to be confused with a real expert