Ammeter Resistance and Voltage drop

I'm troubleshooting an issue with an aftermarket radio for my 1968 Barracuda. The radio is a retro sound radio that requires an always-on 12V and 12V from accessory.

Often upon applying the brakes and/or the blinker, the radio will cut out. I've just verified today with a oscilloscope that the voltage on both the "Always on" supply and the "accessory" supply drop down to 8-9V momentarily (maybe 50 milliseconds, then rises back up to a slightly lower voltage than where I initially started) which I believe is causing the issue (load of the lamps dragging down the voltage). Radio is only specified to work down to 11V.

Both the always on and the accessory supply are attached to the alternator side of the ammeter. I am seeing a resistance between the positive battery cable and the alternator be about 5.3ohms, and a nominal voltage drop of about 1.5V-2V between the battery voltage and the alternator voltage when the car is off. Other than cables, there is only a fusable link and ammeter between the battery and the alternator.

This seems high to me but I don't really know what is normal. I believe my ammeter works, since I can watch it move ever so slightly when the blinkers are on.

I am also seeing what I believe is WAY too much resistance from the starter switch between the alternator side of the ammeter and the accessory output of that switch. I believe this to be my primary issue, because when in accessory or run, I can watch the resistance change between 5 and 50 ohms just by moving the key around. If this accessory output voltage is drooping more due to this high resistance, then maybe that voltage input turning is triggering the turn off. The only thing that makes me question this is I can watch both input voltages with the o-scope, and I don't see one significantly different than the other, the accessory is maybe 0.5V lower at all times,

In short, can anyone tell me what is anormal value for voltage drop between the battery and the alternator when the car is off, and/or what resistance I should be measuring between the battery and alternator? I may have both problems here.


In summary, with car off
Battery Voltage: ~12.5V
Alternator Voltage ~10.5V
blinker or brake lights drop alternator voltage to ~8V temporarily.

Thanks.