For me the red diode was connected to the interior lights.
I guess if everything was installed properly and never messed with in 40 years it's the blue. I guess I didn't bother to check what was proper because I knew that those diodes are internally all the same except for the white ones. For people that don't know the diode module can be disassembled very easily that means not cutting it open so the diode inside can be replaced for just pennies.
Dave B.