I also have a LED strip leftover. I cut it to 24" and was about to mount it to the step frame just beneath the door sill. This would illuminate the step nicely, but the step would shadow the ground and also a step platform if placed there, I nixed that and ordered a 14LED truck clearance light. My original step light had water in it and is badly corroded. A new one would probably cost around $24 with shipping.
Controlling the light is another thing. You can what's called "OR" the light on, by putting a diode in series with each 12V source, tie all the diode cathode together and connect to the light. That way you can have the Porch Light, OR the Aisle Light OR the Step Controller turn the light on. The diodes protect each source from being directly connected to another source and the possibility of one source blowing a circuit of another.
Gale & Hank- 2012 Berkshire 390BH