300TDi Alternqtor

I’m a bit stumped and would appreciate some assistance. The battery light of my 97 Defender 90 recently started glowing, but the vehicle kept starting and the headlights stayed bright. Nonetheless, I replaced the alternator with a John Craddock spare, but the problem remains. On examination, the output on the B+ terminal is a bit over 13V with the engine running, but the voltage from the D+ terminal to earth on the chassis is 6.5V. I’ve tested the continuity of the brown/yellow wire from the D+ terminal to the block connector on the bulkhead, and it is good, with no earth leakage. Is there a diode or resistor that I’m missing, or some other issue?

Thanks in advance.