A 1996 truck does not have that much in the way of high tech energy consuming electronics like todays cars. Do you have a meter such as this one?
If so (it can be any meter, just has to have a seperate amp lead hook up) set the dial to the amps setting (red arrow) don't dick around with milliamps, that's a complete waste of time on a truck this old. Put the black lead in the normal ground connection and put the red lead in the amps connection (green arrow).
NOTE: AS SOON AS AMP DRAW TESTING IS COMPLETE MOVE THE RED LEAD BACK TO THE POSITIVE CONNECTION OR YOU WILL BLOW THE FUSE IN METER IF HOOKED ACROSS THE BATTERY TERMINALS
Now you need to isolate the batteries due to it having two. In this case disconnect both battery grounds and hook your meter up as pictured to the passenger battery.
As soon as you make the connection, look at the meter reading it should be next to nothing. If it indeed is 20 amps or more break the connection. My meter is rated to read 15 amps and I have seen 30 amps briefly when an abs pump shorted out. If the reading is low, turn the park lights on. This should bump the reading to around 6 amps and park lights should illuminate. This just verifies your hooked up correctly.
The factory spec. is 20 milliamps and your meter will read 0.02 amps. Continentals were allowed 50 milliamps due to the onset of multiplexing and advancing technology.