[chengny]

On your truck the black box with the Casco on it was what passed for the "ECU/ECM" (computer). It had only one function. As jetmech85 suggests, it controlled the A.I.R. system diverter valves under certain conditions. It had two inputs;

1. Engine speed - it used the white wire that is lead out of the distributor cap (the same wire is used to drive a tachometer)

2. Incoming combustion air temperature - measured by a sensor that mounted through the side of the air cleaner.

Since your A.I.R system as been stripped, it can be removed.

On your tail light issue - just be sure that the correct bulbs are installed. You'd be surprised how often a single filament bulb gets jammed in to that dual filament socket.

Are your license plate lamps and rear markers working? If they are, that would indicate that power is available in the rear lighting wire harness on the brown wire (feeds the plate, rear markers & rear parking) and the problem is within the dual filament socket/bulbs.