Look in the owner's manual index for fuses. I think you will find more than one fuse block.
On my 2005 ENVOY XUV the stop lamp fuse is located in the fuse block located in the engine compartment. It is fuse #12. In my owner's manual it is labeled "stop Lamp". Within the engine compartment, the location of the fuse block may be different depending on the engine installed. It is there. In my ENVOY the location is between the battery and the firewall. Access is very open, lots of room to work. There are two covers that must be removed to gain access to the fuses. The under seat fuse block has several fuses having to do with the rear lights being used under different circumstances than stoping.
I see in my owners manual the #51 fuse mentioned in your post. This tells me there may be a good chance fuse #12 in the engine compartment fuse block is the fuse you are loocking for.
2005 Newmar KSDP 3910 + GMC ENVOY XUV 37K lbs Moving Down The Road
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