music performance collapses without ignition….

servus, so so far I never had problems with my music: radio gets juice from an additional battery, just like the 4 end stages. has always been great, then I came up with the idea that 4 end stages at a remote line is probably n bissi much and I have an remote from the radio first time n relay, this now switches the juice of the kfz battery, because I assumed that remote and plus also in the end stage are separated galvanically, everything else would be stupid, otherwise the dinger always on… so…so as long as both batteries are full everything goes as usual. the additional battery is switched on by relay when the lima is running, I have checked several times, that does exactly what it should… so, but now the additional battery is almost empty, and the voltage breaks not only the end stages go off, but also the remote signal collapses?!? -don’t understand weshelb, because the front battery from which the juice comes still has solze 13V… as a result so, no more music. now thought is: turn on the engine, then bring the lima juice and the relay switches the rear battery to the kfz circle, then music should come again, BUT the music came before, if I only switched on the ignition…and it was always….I understand the world nbcihtmore….then I would have to have a rrelais or something else sitting somewhere, which closes the two batteries together when the batteries are lit?!? -But didn’t I have any more music? t….