It is the self contained 18ACR style.
Been very weird on the way home. Light was on entire journey - about 30% illuminated on start up. Sidelights on, and it went to about 60%. When I gave the windscreen a wash and wipe, went to about 80%. However, when I got to about 3000rpm, it started pulsating/flashing between 60% and 30%. and then would stop when the revs dropped.
Charging was consistent throughout journey, and didn't fluctuate when the lamp was flashing!
If you can get a multimeter on say the 10 volt DC range place the negative probe on the ignition switch side of alternator warning light and positive probe on battery positive or as close as you can get to it, even the ignition switch itself. With engine running the reading will show the volt drop you are getting which is driving the reverse current through the warning light. If my suspicions, based on your findings above, are correct you will find this volt drop due to resistivity somewhere in feed to lamp, increases with extra loads. The pulsating you got is likely due to alternator regulator action.
On a voltage range accepting up to 15 volts you could also measure the voltage between alternator side of warning light and ground. This should be very close to battery volts accepting it will rise with engine revs above idle. If so the fault is definitely due to a significant resistance in wiring between ignition switch and warning light. Please let us know how your investigations progress.
BTW the seat belt warning diode appears to be a red herring. It's there to stop back feed from shared buzzer triggering seat belt warning light when door is open as per below.
![[Linked Image]](https://www.mgb-stuff.org.uk/images/sb1.jpg)
