This problem will exist if the rear door is ajar or the rear door light switch is broken or grounded out .
This is also a safety feature on disco,s to warn people you are opening the back door while the engine is running on the side of the road .
Basically a warning to tell you you left the back door open .
I had this issue on a Suburban. Ended up manually closing the circuit to get rid of the probalem as I could not get a new switch that was causing the problem. GM decided that the rarity of the barn door suburbans weren't worth manufacturing speacialy parts for.