I am thinking it would be blue book...
A black book is the often mythical book guys supposedly have full of girls phone numbers who are just waiting for a call.
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Check out the
Kelley Blue Book website. Put in your 92 Cab in used car values and you should be able to find values based on what you would expect for a trade in (lowest), expect if buying from a private party, and what you would expect to find retail based on your location, car options, mileage, etc etc.
In Seattle, for a 92 Cab with 33k miles in excellent condition, values for the above would be $13k, $16.5k and $20k respectively (in US dollars). But again, it all depends on your location and options.
Blue Book values are used by insurance companies to determine replacement cost, and therefore at what point they would "total" a car in the case of an accident (Blue Book cost of vehicle is less than cost to repair).
One other thing to think about is if you have financed your cab at all, and if your total financing might exceed the value of the car (car is worth $8,000 and you owe $10,000 for example), is to ensure you have "gap" insurance. This covers you for the difference between what the car is "worth" and what you owe on the loan for the car (in this example is would cover the $2,000).
Hope that helps some!
Chris