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Another thing might be some old switches have the resistance start wire
Which goes to coil + to bypass ballast resistor and that causes issues

But if u fixed it by running a relay sounds like feedback thru acc circuit
Only thing I run off acc circuit is stereo stuff
Everything else goes thru ign circuit
I run 50 amp wire for ign circuit and that normally does the job
Enough for wipers and heater and gauges and blinkers and rev
Everything else runs a relay

If u do get time to test it more
Pull out all acc fuses then add one at a time till u get motor cranking again when switching off
But this means putting it back to how it was when it played up

Good it's all sorted now