Good reason. There is no speed sensor. Vehicle speed is computed by a Hall
Effect sensor mounted in the rear differential. That , in turn, feeds the
ABS, and the chassis computer where the pulse train is measured against time
and then divided by a series of divisors for the various outputs like the
speedometer and display computer overhead. Normally, the GPS itself measures
speed (VMG) by calculating position change over time.
Steve

I see a check engine lite coming on soon!!

Glenn Beasley
Chrysler Tech

Okay...  now that all the histrionics are over with, let's see if we can
actually offer some assistance.

The Pioneer unit simply wants a pulsed input from the rear wheel speed
sensor.  While not absolutely required (as it can determine speed via GPS),
it does help the system respond more quickly to changes in vehicle speed.
Given that it's designed to go in to a multitude of vehicles, it will
automatically calibrate itself to determine the pulse-per-minute to vehicle
speed ratio (again, using the GPS readings).

So, in short, all you need to do is tap the rear wheel speed sensor wire
before it goes into the ABS controller.  Unfortunately, the 2008 wiring
diagrams aren't yet available on Dodge's site
(http://www.dodge.com/bodybuilder), so you're going to have to wing it with
the wiring.  Examine the connector at the rear differential, identify the
colors of the wires, and find them up at the ABS computer connector.
Assuming they didn't change anything from '06 to '08, you're looking for pin
#42, a 20-gauge dark green/white wire.  That's the positive side of the
speed sensor, and probably what the Pioneer is looking for.