Amazon: '66 Amazon Headlight > brake-light : gremlins

[BLS07]

Hello fellow Volvo enthusiasts !

Would greatly appreciate the imparting of some advice in respect of the following conundrum, please.

Vehicle spec:
Amazon '66 122, LHD, B20, and some months back I replaced dynamo with a nice alternator upgrade. All has been brilliant since that swap.

until..

Headlights had been fine, until an issue with original foot switch arose, currently working on getting that out of the car. Screws have ceased, and easing them out is taking a while.

I have an interim replacement in the car using a foot-dipper switch from Brookhouse Volvo. Which itself seems to be A-OK.

Standard wiring throughout the car, per Haynes and original Volvo manual resources to hand.

At foot-dipper switch, wiring is:
i) Two red, one larger gauge than the other. (main / high beam)
ii) One Grey (dipped / regular beam)
iii) One Yellow (power from switch)

Problem / Issue

1) Headlights are only both on when foot-dipper is set to main-beam ("high beam").

** Only front left headlight (and all til lights) on when foot-dipper switch is set to "dipped beam"

2) Brake-lights only "pulse" / "flash" when brake pedal engaged IF headlight switch on the instrument panel is OFF. (all the way in)

3) Tail lights all ON and A-OK when headlight switch on the instrument panel is ON. (just brakes don't "flash" per 2 above)

4) Indicators - all four work fine, all the time.

5) Front left parking light - "glows" when brake pedal is engaged. AND when regular-beam (dipped beam) is operating per 1 above.

Is this likely;

A) Something in engine bay section of wiring (connector / junction box or relays)

B) Something in the Grey wire from foot-dipper switch to the junction box

C) something else entirely?

Appreciate wiring / electrical fault tracing is oft more complex than this - but just wondering if there is an obvious answer that as a newbie to all this am not picking up on.

Would really appreciate your advice !

Cheers, BLS.