Different stereos have different frequency settings, but generally they consist of high and low shelf EQS for the "treble" and "bass" controls. A good place to start might be 150-300Hz for the low shelf and 4000-8000 Hz for the high shelf (totally an estimate as these frequencies could vary depending on the unit).
More elaborate aftermarket stereos will vary depending on the make/model, but you might be able to find more information on EQ points/crossovers/etc. in the manual.