Is this on an ST220?
Have you tried it with the light units totally disconnected to rule out a fault in an actual lamp like corrosionborvwater ingress.
If it's Fuse 80 on the ST220 then it's just for the reverse lights.
You could have a fault maybe relating to the parking sensor control unit if one is fitted, try unplugging that. Alternatively has soneone at some point fitted an (illegal) towbar & butchered the wiring?
Failing that you've got a short somewhere, luckily the Wiring on the ST220 is easy to get at & trace.