If you can't tap the back side of the coil why not tap and split the lead going into the coil and run that to the brake switches it should all work the same it's not going to use the coil to run the light bulb should be no problem.
Instead coil lead will go through the light bulb then to the coil when the brake is applied.
And you will still have the safety feature if the rear ball blows the engine will die when the brake is applied and you don't need to buy new switches.
If your condenser is bad and not suppressing the voltage Spike that might blow the tail light out but other than that I don't see any problem.
But I think your real problem is you just have a bad ground which happens a lot on 103 s you need a nice Flex ground lead coming from the engine case to the frame and headlight bucket.
That will stop the tingling and Spark off the choke cable obviously that's an indication of a bad ground or weak connection.