Dodge dakota

Most likely have a dead short somewhere and only way to find that is through trouble shooting. Pull fuses for non essential circuits (like lights) and see if problem goes away. Check over battery cables, follow them to starter and alternator, and make sure there are no bad or frayed connections. Look for any wiring insulation that is stripped or frayed. Wouldn't hurt to have charging system tested to make sure battery and alternator are good. Batteries with a dead cell or loose terminal can work one moment and be dead the next.

Clearly KenF, is shooting in the dark and probably hasn't owned one of these trucks. I can almost guarantee your problem is the voltage regulator. Unfortunately in these trucks it is located in the ecu. So there is 2 ways to fix this. The first is to go buy a new computer. The second, is to buy an external voltage regulator out of the older model and wire it in. It only cost about $20 and an hr or so of your time. I just did this on my 91 dekota and now it runs and charges just fine. GOOD LUCK!!