Your symptoms do indicate a high resistance connection. It could be on the ground or hot side of the circuit. There is less involved with multicircuit grounding as they all ground to the chassis or engine. The battery grounds to the frame and there is a heavy ground wire from the bellhousing to the frame to link it all up.
The power side goes from the battery to the starter and is extended first through a buss panel that should mount to the pass side frame rail. The large red (at one time) wires carry unfused power to the light switch and from there to the un-fused sections of the glow circuit and ignition switch.
Get a test light or volt meter and apply the load and follow this path untill you loose battery voltage. Check first at the battery pole, then the battery cable, then the starter terminal, then the philips screws on the buse bar, then the light switch, then the ignition switch.
From the other side ground should look like ground at the battery pole, then the terminal, then the frame, then the engine block.
Bosch Master, ASE Master, L1
33 years MB technician