You should be able to get a reasonable resistance measurement, by simply measuring between CCS and GND with the module shut down (that old a vehicle, it'll probably shut down pretty quickly, but would still be wise to give it a couple minutes with the ignition off before measuring).
Try measuring the resistance with a multimeter connected both ways, in case there are any diodes that may upset resistance readings.
Once you know the internal resistance of the module, some series resistance calculations should give you the resistances needed.