The EEC module is the Evaporative Emission Control (also sometimes called the Electronic Engine Control). It does a number of things, but its main purpose is to adjust the air/fuel mixture to maximize fuel efficiency.
The "Related Link" below has extensive information, illustrations, photos and diagrams for the 1990-1995 heating/cooling systems. This does not look like an easy under-taking, with all the airflow directions being controlled by separate vacuum controls. Oh yeah, and the dash has to come out.
You need to take it to a Ford dealership with service department, they can then put your vehicle on a machine that they hook up to your vacuum system and pump smoke through it to find the leak. Usually cost like $100.00
Check the emission label usually located on the top of the radiator or on the core support just in front of the radiator. You will see a diagram of all the vacuum lines and emissions equipment. Generally-the EGR valve is located near the throttle body.
When fixing or changing a radio in a car, a wiring diagram will help make sure things are connected correctly. A wiring diagram for the radio of a 1990 Dodge Dakota can be found in its maintenance manual.
Behind the engine on the passenger side, should have a yellow loop handle on it. Many times they are also found in the back-right side of the engine compartment. The Owners Manual should have pretty good instructions on this - finding one that old might be a problem.
There are two. One is inside near the brake pedal. It is tucked up under the dash, but you can pull it down to access it. There is another bank of fuses inside the engine compartment. on the driver's side, up against the firewall, right on top. steve phx az