In all honesty, I can't see why installing the AS Piper Cheyenne could cause the Nav and Strobe lights on the wings of the 757 and 767.

CS757:
[LIGHTS]
light.0 = 9,  67.00,  -2.00,  5.19, fx_vclightwhi
light.1 =10,  67.00,   2.00,  5.19, fx_vclightwhi
light.2 = 7, -17.00, -47.00,  2.50, cs757_sf
light.3 = 7, -17.00,  47.00,  2.50, cs757_sf
light.4 = 8,  13.40, -17.47,  3.40, cs757_edge
light.5 = 8,  13.40,  17.47,  3.40, cs757_edge
light.6 = 5,  23.50,  -7.00, -3.50, cs757_edge
light.7 = 5,  23.50,   7.00, -3.50, cs757_edge

CS767:
[LIGHTS]        
Types: 1=beacon, 2=strobe, 3=navigation, 4=cockpit, 5=landing, 6=taxi, 7=recognition, 8=wing, 9=logo, 10=cabin
light.0 =  9,  70,  -2, 5.2, fx_vclightwhi
light.1 = 10,  70,   2, 5.2, fx_vclightwhi
light.2 = 7, -29, -58, 3,   cs767_sf
light.3 = 7, -29,  58, 3,   cs767_sf
light.4 = 8,  21, -19, 3,   cs767_edge
light.5 = 8,  21,  19, 3,   cs767_edge
light.6 = 5,  32, -8.4, -6.6,  cs767_edge
light.7 = 5,  32,  8.4, -6.6,  cs767_edge

It would mean that the installation of the AS Cheyenne deleted the cs757_xx and cs767_xx lights. And that's only if the position lights are not built into the model themselves.

Mark