Heres what Im getting ready to do, at the moment my running lights are on with my high beam switched on thru a relay triggered by the blue wire in the 3 pin connector, Im installing a on/off switch between the keyed on brown wire in that 3 pin connector and my relay so my lights can be on/off with either hi/low, Im doing this so my running lights arent on with my high beam as it modulates unless I want them on. Im thinking this will work.
Maybe I'm misunderstanding you here.. but.. the end result you're after is having your running lights ON when your low beams are on, and on HI only
when you want them to be?
What if you used a 5 terminal relay (with an 87a - HOT when UN
triggered) where your trigger is the high beam?
When on LOW beam the terminal 87a would go to the running lights since you're untriggered, but when you trigger the relay with the highbeam the regular out, Terminal 87, goes hot to the running lights. You'd have your switch on this line from 87 to the running lights.
I only see two (actually 1.5..) potential issues with this:
1) Your power IN has to be on a relay so as to have the lights go out with the headlight when cranking. This would depend on how you wired your aux fuse block I suppose. I don't know if the way the initial picture has it set up will allow that (unless the license plate light goes out when cranking). You'd either have to have a separate relay wired for regular running lights "always on but off when cranking" to put power IN to pin 30/51, or have your fuse block triggered by something keyed-on, but off when cranking.
2) You'd have two "hot inputs" into your running lights (87a and 87). Since this is coming out of the same relay, and can't [read: shouldn't] be hot at the same time this is a moot point. But for safety redundancy to avoid back feeding your + through the other pin on the relay you might be able to rectify this problem through the use diodes to control the flow?