This is probably the easiest way. When you apply 12v to switched input you should see resistance decrease to 0 and have continuity.
2001 540i - Orient Blue
M5 bumpers, wheels, mirrors
S/C, water/air IC
A1 Headers, Random Technology Cats, Eissenmann Exhaust
3.15 diff, evosport power pullies
Koni Sports, Eibach Springs/Sway Bars