Not sure what circuits are what for the SL-C ISIS system, or what you use for instructions to hook them up, but you should be able to start with the basics and use a DVOM (not a test-light) at the input wire to the Mastercell to verify that the proper input is indeed being pulled to ground when the park lights are turned on at the switch. Once you verify that is happening, if the programming is correct, every time you turn the switch on and off, you should see a corresponding LED come on bright at the Powercell. If I'm thinking right, the output LED on the Powercell should glow dim when you have a bulb/load/test-light hooked to it, and should glow bright when the Mastercell is commanding that output to turn on. If you don't have any load hooked to the parking lights output wire, the LED will not glow at all. Without a spreadsheet to tell me what all the inputs and outputs are programmed to do on the SL-C set-up, that's about all I can tell you.
