It's a relay (auto shutdown), it kills power completely to the ignition, fuel pump and injectors when the key is off, it's also designed to kill power under certain circumstances to prevent damage, fires, etc.... On your 91the ASD is located in the underhood fuse box on the left fender well
Search Dodge Dakota Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
Dodge Dakota Experts