Thus far I have reworked the original horn switch to allow isolated current when switched rather than earthing as it did originally.
Not easy to see from the photo but the switch had to posts, one originally was blanked and the single pole earthed to the body of the switch. I drilled out the brass rivet that earthed the switch and soldered a new trace to the disabled pole from the switch contact and epoxied in place to allow current to flow from post to post.
Once that was out of the way I protoyped up now revision two of the circuit to detect switch state via a simple three wire circuit to the Arduino. This uses 5v / Ground and single analog input with resistors in parallel. This allows detection of multi switches being pressed (ie indicator and horn) also with different value resistors being used for each switch in the housing.
Next is working with resistor values to allow accurate detection of multi press and start testing with canbus messages to other Arduino which is controlling a relay board at the other end.
