That really is an unusual problem.
Because both brake lights (both or either) cause the relay to close, it should really not be possible for only one brake light to cause any odd behavior on the 'other' side of the relay; that is actually what relays do, they isolate one circuit from the other, so the two brake circuits should not be able to have different results on the other, or power side, of the relay.
So, the only thing I can think of, especially since you mention this changed after you changed the relay for another one, is that the relay is not closing and staying closed, as it should, but instead rapidly opening and closing. This is called 'bouncing' in the electronic community and resembles 'chattering' or buzzing as the relay rapidly opens and closes. The usual cause of this is insufficient current going through the relay's coil to keep it closed, and that is caused by (almost always) a bad connection on the power side of the relay.
I would suggest not using a frame as a grounding point but instead tap into the actual ground circuit on the bike. There will be a lot of wires used for ground all over the bike, find out what color they are (they will all be that same color), and tap into one of those lines for the ground connection of the relay and the control pad but NOT the ground from the Rostra's actuator. Even better would be to run a wire of at least 16 gauge directly from the negative terminal of the battery and use it for all of the ground connections of all circuits installed for the Rostra, including that relay.
If that fails, though I really think that is the problem, then I would check the rear brake light circuit to see if it is chattering. This can be tough but using an analog voltage meter may show the needle jittering slightly if checking the voltage on the brake circuit when it is on though decent quality, taut- band analog voltage meters are somewhat expensive. Best is an oscilloscope but most people do not have those lying around.
Still, I suspect nothing but a ground problem, especially because you are using a frame grounding point and an actual ground line will fix your problem I think. Besides that, it is a good idea to do it that way anyway so it would not be a waste of time in my opinion anyway.
Best of luck with that.
Brian
Apologies in advance but I found my way to your forum whilst researching my install and now I have a problem was hoping the obvious vast experience within this site might be able to help me? Ive searched extensively but cant find an answer so I was wondering if you guys have ever encountered anything like the following?
Ive installed a 250-1123, using a CCS100 keypad, on a Hayabusa gen2 setup for touring (pause while howls of derision die down 😉).
It all works perfectly except for one strange trait.
If the unit is turned off, and I apply the rear brake, then when I release the rear brake, the unit turns on - intermittently. My install uses the relay-brake-light option as The gen2 has a LED brake light.
I dont have the issue when I apply the front brake.
The red wire and the activation for the brake light relay are connectd across the rear brake light switch, but front brake switch is in parallel to the rear so it seems odd I only have issue with rear brake.
I Originally had the issue constantly but I swapped to a different relay and now the problem is intermittent.
Both the grounds that go to the relay and the overall ground for the Cruise actuator go to the same bolt on the subframe.
Does anyone have any thoughts on what may be causing this annoying trait? Maybe I need a diode or capacitor on the relay somewhere (really guessing now)?
I am aware of another install that had a similar issue with an ungrounded brake light modulator installed but my brake light is stock.
Any incites would be much appreciated.
Cheers Chris