Dodge Repair: 98 Dodge Dakota

Question
Heater core went.  My son bypassed the core but in doing so the speedometer does not work.  All other instruments and the radio still work.  What could be the problem ?

Answer
Hi Bill,
I don't have the wiring diagrams for that year, but I wonder if it might be that an internal circuit board solder joint loosened up which connects the speedometer 'head' itself to the circuit board.
By '98 I would believe that the speedometer info which comes from the output speed sensor of the transmission goes to the transmission control module (assuming an automatic transmission) is digitized and goes to the power train control module and thence to the instrument cluster in a digital data wire that carries all the info for the cluster. But without the wiring diagrams I am really only able to guess. I find it hard to believe that there is a dedicated single wire for the speedometer proper.
However there may be an instrument cluster self-test that will give you a fault number. With the key in the 'off' position press and hold the trip odometer reset button, turn the key to the run position, then wait to see the word "chec" appear in the odometer window, then release the button. Then watch to see if there are a series of various test done, and notice when it is over if there is a 3-digit number in the odometer window. If so let me know what it is.
The above is based upon the '04 manual which is the earliest that I have.
Please read the PS below and respond to it.
Thanks,
Roland