Testing everything again and I noticed something important...

The LEDs in the dash flash momentarily when the horn button releases - not when it is pressed. This also occurs with the original horn installed and no relay connections. Swapping the polarity of the LEDs makes no difference. there is no flash when using the original bulbs.

I couldn't find the earth for the dash/handlebar controls to check that.

Sooo ... the flash is nothing to do with the relay. Seems to be the LEDs - sensitivity?

Any thoughts? OK to just ignore the momentary flash?

Cheers, Richard
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09 America, Staintune Pipes, K&N, Breathe, Hagon Nitros, AI & O2 removed, tune 20184 (modified), MTX-L a/f gauge