On my 93 240 wagon, I have a long standing issue of apparent low voltage and electrical load sensitivity on interior circuits only. This issue worsens as the car gets fully warmed up. The symptoms are:

1. Dash mounted VDO voltmeter drops to 12-13 V after starting at 14-15 V.
2. Dash light brightness flickers randomly, and dims notably when a high load accessory like the blower motor, wiper blades, or turn signals actuate. I also see extreme dash voltmeter jumps downward when large accessories are used.

Generally, as loads increase (headlights, foglights, etc), the voltmeter drops to 12 V or sub-12 V levels. Levels that would make you think you weren't properly charging the battery.

When this issue occurs, I can put a calibrated voltmeter directly on the battery terminals and read 13.8 - 14 V steady. The voltage fluctuations in the cabin appear to be independent of the true charging voltage on the battery (engine running).

I was running a 100A Bosch alternator, but as a sanity check, I installed a Denso 100A unit and got the same results. Steady charging voltages at the battery, but low voltage and load sensitivity inside the cabin.

I'm thinking this looks like a bad or partially bad ground somewhere. Is there a ground tree located somewhere that could cause this issue just on interior circuits, but leave the primary charging circuit alone?