I don't think there is a relay for the reverse lights, it's just a switch in the transmission and a fused power from the fuse box.
When the vehicle is cold, pull the switch out and actuate it manually while watching via a mirror to see if the lights come on or not.
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"¿ʞɐǝɹɟ ɐ ǝɹ,noʎ uǝɥʍ 'ʇı ʇ,usı 'ʎlǝuol s,ʇı"
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