To be certain, you should:
* Consult your owner's manual: This is the best source for the precise location.
* Check online forums: Search for "1990 Dodge Ram Charger ECU location" on forums dedicated to Dodge trucks and cars. Someone may have pictures or more specific instructions for your model.
* Look for a black box with numerous connectors: The ECU is usually a fairly large black box with many electrical connectors. Look carefully under the dash on the passenger side, focusing near the firewall and inner fender.
Remember to disconnect the negative battery terminal before working near any electrical components.