QUESTION:
Does Radiance take its solar position and sky illuminance from ECOTECT or does it calculate it's own ?
ANSWER:
It can do both.
Unless you specify the values to be taken from ECOTECT in the Radiance export dialog box, Radiance will generate it's own sky illuminance and sun position from the current Lat., Long., Date and Time in ECOTECT.
Not sure what formula Radiance use for sun position, but it is likely to be slightly different from ECOTECT as there are many different formulae that are used for calculating sun position. For your reference ECOTECT uses the formula from Carruthers, Uloth & Roy 1989.
For the sky illuminance you can see how this is being specified in Radiance by opening the sky.rad file in notepad (located in the same folder where you are saving your Radiance export).