That would be ground theft happening.  

I would first check the other bulbs, those 1157 bulbs have a sneaky way
of the glass coming loose so the filament wires behind it can cross.  I
even recently had one that just plain blew and the running light
filament fell across the brake filament causing the same thing.

One like that in the front will cause the same in the back.

Then if all the bulbs check out good, I would be looking for the running
light ground.  It will be a wire at the end of the harness likely.  They
use a cable and the brake filament uses the fixture to ground.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's
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