OK... so here's the theory (Geoff, pretty sure you know all this, but gonna post it anyways in case others are curious.)
Basically the VSS outputs a pulse every time the driveshaft turns a certain amount. The ratio of turns to pulses depends on the speedo gear you have installed. Anyways, the faster the driveshaft is turning, the faster the pulses get.
Really what you want to do is turn the lights on when the pulses get closer than a certain number of milliseconds, and then turn them off again if the pulses get slower than that. So the first thing to do is design a circuit that will measure out the time between the pulses. I decided to use a "one shot" timer - basically, you give it a pulse as an input, and it sends out a voltage for a predetermined amount of time, no matter how long the input pulse was.
Next, we need to turn the lights on whenever the next pulse arrives before the timer expires. So wire a flip-flop (a circuit that will remember the state you left it in - you send it a voltage on the Set input to turn its output on, and then another on its Reset input to turn it back off) with some logic that will trigger it to turn on if the one-shot timer's output is still on and another VSS pulse comes in, and trigger it to turn off if the one-shot timer's output is off when the next VSS pulse comes in.
But... since the real world varies slowly, and digital logic chips don't, if you drive along with this setup at EXACTLY the speed it's set to trigger the lights at, they will flick on and off very annoyingly and randomly. So we need another one-shot timer that's set for a slightly different (longer) pulse time, and wire the flip-flop to reset when the turn-on timer is off, the turn-off timer is also off, and another VSS pulse arrives.
Block diagram (still needs some details added):
Bill of materials:
2x 74LS121 one-shot timers
1x 74LS76 JK flip-flop
2x timing resistors (unsure of value, may be different, will have to calculate this later)
2x timing capacitors (see above)
1x 0.1uF 16V (or higher) tantalum decoupling capacitor (niobium oxide and other dry electrolytics with low ESR are also suitable)
1x 1.0uF 16V (or higher) tantalum decoupling capacitor (niobium oxide and other dry electrolytics with low ESR are also suitable)
1x VN0300 or 2N7002 N-channel MOSFET
1x automotive relay
1x 1N5819 back-EMF shunt diode (or use auto relay with built-in diode or resistor snubber)
This whole circuit (minus the relay) should be powered off the sensor power circuit - draw power from the VSS power line and it'll work great. Go over 5.5 volts and it WILL go up in smoke; if you want it to work off standard auto 13.8V power I can add a voltage regulator to the design. The way I've designed it so far it might turn your headlights on when you start the vehicle, or it might not, it's left up to chance as the flip-flop will choose randomly when you power it on. I can fix that but it'll add another part to the list... also, the two timers (marked "on" and "off") are actually the 74LS121s, but require more detailed wiring (adding the two resistors and capacitors and setting some option pins) but I haven't figured that part out yet.
If I find time Monday or Tuesday I'll finish that part of the schematic and throw together a quick PCB design.