You could use the digital phase detector (phase comparator II) from a 4046 phase-locked loop circuit. It also has a VCO oscillator you can use to provide the 1.5kHz reference frequency.
Instead of connecting the phase detector output to the VCO input as for a normal PLL connection, just use the phase detector output to control the relay (the output will require a MOSFET amp to drive a relay). If the input frequency is lower than the VCO frequency the phase detector output is low, if the input frequency is higher the phase detector output is high.
Note: the detector output is a series of pulses so you will need to add a small cap to ground at the detector output to get a steady dc signal.
A pot connected to Vdd and ground with the wiper to the VCO input will control the frequency of the oscillator to give the adjustment you want.
The data sheet will show you how to calculate the resistor and capacitor values to get a nominal 1.5kHz VCO frequency.
And, of course, if it's to be powered from the car battery, you will need some type of transient circuit to limit the high voltage spikes that can occur in a car electrical system during starting, etc.