Speed relay?

If the vehicles have an overdrive (and a signal from such being engaged), then this is likely the easiest way to pick up a trigger signal. 

When the signal becomes 'active', it can be used to turn on a solid state relay (directly or indirectly) which then applies 12VDC power to your apparatus.  Although the amount of current required to turn on a solid state device is generally quite minimal, some concern should be given to what the signal is driving upstream, unless it is just a light - then no problem.  It sounds like you may have already had some thoughts in this direction.

The first thing you would need to determine is what the 'active' state of the signal is when the OD switch closes - it will apply either a ground or +12VDC to the signal line.  That would determine which type of solid state device you should use - meaning a 'NPN' or 'PNP' gate arrangement to turn the SS device on.

If the OD switch applies a 'ground' (would be my guess), then a "PNP' type of gate is in order.  When a negative signal (i.e. ground) relative to the collector is applied to the base (collector has a constant '+' voltage at some level), the device turns on & applies power to the equipment.

If the switch happens to complete a circuit to +12VDC, the same scenario applies except you would use a 'NPN' gate arrangement on the input.

A few comments ... 

1. Unless you choose a device that is designed to accept a +12 to +15VDC input signal state, you'll need to wire a resistor in series with the base to protect it from an overvoltage condition.  Standard TTL logic devices typically have a 0V (ground) to +4VDC operating range.

2. If you have someone collaborate on a design, make it so the unit will accept either a ground or +12VDC signal input.  That way if you change vehicles, and they did it 'the other way', then you're covered.  Just move the connection point.

3. If you want to get a step fancier with the setup, do the design such that there is a 'holdover' period of some duration before the power to the equipment is removed.  If you catch passing gear or the transmission upshifts for a while, then the OD signal likely will be temporarily removed.

This could be done with a simple circuit using an potentiometer (variable resistor) and a capacitor with the right IC to provide an adjustable holdover period (555 timer, etc).

Radio Shack may have something already canned that could be used as is, or with a little modification.

4. Wire a control switch in so you can shut it off it you need to.

Trying to use a signal off the magnetic pickup for the cruise control would be considerably more involved, not to mention a bit risky.  Not only are the pulses generated by the MPU of a much lower current and voltage, you'd then have to have comparator circuits to determine when to activate the equipment - reinvent the wheel.

About the only other alternative I can think of is to get a 'Dear' sensor like I have.  She'll warn me about everything from a frog in the road to airplanes landing in a 10-mile radius.  "Yes, Dear.  I see it, Dear."  Phbbbbbbbbt!

Kinda reminds me of an old '67 Ford Fairlane I owned that didn't have an Intermittent function on the windshield wiper.  So, I just cut into the horn wire and added a single pole, double-throw switch with a couple of diodes, and tied the horn button to the wiper motor when it was raining.  Tap the horn, get a wipe.

Told Sweet Thing about that switch 50 times, I know.  Came home one day and she started raisin' Holy H... about that "Stupid" car of mine that makes the wipers jump every time you try to blow the horn!  And handed me the broken off horn ring to prove her point.  Ah, well.  Thank God for modern conveniences.

they do make deer whistles that work with the wind you know.

they look like a little hollow tube that just sticks on to your front bumper.

the tube creates an ultransonic sound that we cannot hear.

check into a sports/hunting supply shop.