Is this high frequency filter schematic correct?
Nope this circuit is not correct. You've shorted the node between the resistor and capacitor to ground, so the 'signal' at Aftermarket speedo will be at ground potential.

If you want to filter out the HF content, you want a low pass filter. This can be made with a resistor and capacitor, but wired as in the schematic below.







simulate this circuit &ndash; Schematic created using CircuitLab

The cutoff frequency of this filter is \$1 / (2 * \pi * R * C)\$ and using the values of 10uF and 100r from your schematic you would get a filter with a cutoff of 159.2 Hz. Content above this will be attenuated at a rate of 6dB per octave.