It looks like you already have a solution that will work, but keep in mind that if you have a GEM at all, there's probably a way to do it without a separate timer module, assuming your relay box has contacts for the defroster relay and you install one. That is, I am assuming the same GEM is used and on a trim level that doesn't make use of all possible options packages, those GEM controls were going unused.
Per that diagram I posted previously, you'd wire the GEM connector C280 pin 1, to the defrost switch, then the switch to ground. With a factory switch/light assembly you'd also wire the relay output going to the defroster circuit/mirrors, through the switch-on bulb indicator.
You'd make sure you have a 10A fuse 32 installed, and wire from the relay output to the heaters and switch-on bulb. Note that what I am referring to as a "bulb" is really the resistor and LED in series in the diagram, not the other bulb that lights with interior lighting on.