OK - took the switch out, and in fact there are 3 wires plugged into it - white, green, and dark blue. Unplugged all three, and tested the switch with ohmmeter. Top and bottom terminals (white & dk blue) are connected regardless of switch position. Middle (green) is connected to both other terminals in the on position, and disconnected from both in the off position. Both white and dk blue are required to turn light on when the main switch is on, and I believe the white must be ground, considering a volt meter shows +13V across white and dk blue with + probe on the dk blue. Shorting across the green and dk blue wires does NOT activate the pump when the main switch is off, and shorting across the white and green also doesn't activate the pump.
Also checked the wire colors on the main switch, and they are white, purple, and red, so that doesn't help trace the problem.
Any ideas ?
2003 Ford F-350 V-10 Crew Cab 4WD Long Bed
2004 Jayco Designer Medallion 29 RLTS 5th wheel
Bill, Gayle, Teddy (Jack Russell terrier), and Honey (Beagle)
Retired at last !
Nothing is foolproof to a sufficiently talented fool.