Followed most suggestions here:
Fuse OK.
Power getting to step mechanism, as the step-light was working (shares power lead to motor).
Tried dragging step out to see if limiter was jammed - no luck.
After poking around & getting nowhere, took Girt to Murray Valley Motorhomes in Strathmerton (90-odd km away - nice outing for the hounds

After Darryl poked around & checked the circuitry (all OK) he took 12v power direct to the drive motor, whereupon it moved the step part-way out while making a 'nasty noise'. He gave the motor a tap with his shifter, and LO! the problem was solved.
He then focussed on the other electrical mods, and wrapped the job up in a day.
Analysis: Probably a bit of grit/mud/other $h!t has found its way past the shielding & into the winding. Tapping the motor casing may have dislodged it.
Darryl had the steps going in-out-in-out for a while without further probs. So have we.
But he wouldn't sell me the shifter...

