First of all, did you use a coolant thremoswitch or a thermosensor? If you really used a thermoswitch, it will have a specific 'on' temperature which is often stamped on the side of the switch. It cant be modified by the addition of external components. If you happen to know the size of the thread and the desired setpoint, I can probably recommend a suitable switch (unlikely to be from a native MB application).

If you've installed a thermosensor, its the wrong component and will never give satisfactory results. A thermosensor has to work with a digital control, not a simple relay. Adding a resistor is just wrong, you're mixing and matching incompatible designs.