with a P-channel MOSFET, the load goes between the MOSFET and the ground rather than how you have it shown. Otherwise, good.
Did you mean that I need to just swap where the 18V and GND are, or should I swap everything that's connected to the drain with everything that's connected to the source?
First, determine the gate voltage you need. This will give you the ratio between A & B, since they form a voltage divider. Since your supply voltage is 18V and -9V below supply is well into saturation for any reasonable MOSFET, you can just set them equal to each other.
So, basically what I want is enough voltage to put the MOSFET into saturation, and to do this, I have to supply some % of the supply voltage (18V in my case)? Makes sense enough.
Second, you need to determine the size of the resistors required to keep from toasting the TL5940. Since it's a constant current output, I don't think you have to worry too much about toasting it during turn-on -- just don't force it to limit the current too much during the on period, since that dissipates heat. If you set your peak current to 20 mA, then V=IR gives you a total resistance of 900 Ohms...
I'm assuming you were referring to setting resistor A and B to be equal to 470 Ohms each, and set the current limiting resistor of the TLC to 2*470 Ohms?
... and finally this configuration will also be able to make the LEDs go completely dark?
Thanks so much, and sorry for the late reply... I'm in the process of purchasing a new truck... I hate car shopping. 