It's the middle one in the picture...Which wire is getting hot?
If it is the red wire to the battery is warm that could be OK, if it is charging heavily. And the regulator will dump current to ground and so it can get hot also.
But check the wiring connections for all that as well as the 30 amp main fuse. Because if it is still the original strip of fuseable metal like mine was, They can crack and corrode and cause a resistance there.