It's located inside the fuel tank as shown in the following picture.
I also want to advise you that there was a recall on that item for your truck. The recall involved a number of emissions sensors It may be too late for you but here is the recall.
August 17, 1998
VOLUNTARY SERVICE CAMPAIGN
1996 TRUCK MALFUNCTION INDICATOR LIGHT (MIL) ISSUES
CAMPAIGN I.D # : P8159, P8160 & P8161
All 1996 Truck (D21) - California & Federal emissions vehicles
Some 1996 Nissan trucks are experiencing incidents involving certain parts of the emission control system which are causing the malfunction indicator light (MIL) to come on. To ensure customer satisfaction, Nissan is conducting a Voluntary Service Campaign to replace certain emission control parts with improved parts at no charge to the customer. These parts include the throttle valve switch, auxiliary air control valve, fuel temperature sensor and air temperature sensor. A water separator kit for the evaporative emission control system will also be installed. In addition, the vent control valve and vapor canister for vehicles in non-sunbelt states only will be inspected and replaced if necessary.
Nissan has assigned identification numbers P8159, P8160 & P8161 to this campaign. These numbers must appear on all communications and documentation of any nature dealing with this Campaign.
NUMBER OF VEHICLES POTENTIALLY AFFECTED
The number of vehicles potentially affected is approximately 84,000.
It is a dealer's responsibility to correct each vehicle falling within the range of this campaign which for any reason enters the service department. This includes vehicles purchased from private parties or presented by transient (tourist) owners and vehicles in dealers inventory.
Attention California Dealers: Although this action is being conducted by Nissan as a Service Campaign, under California law this is considered to be an emission recall. California law prohibits owners from renewing their California registration if emissions related recall work has not been performed. California dealers are now required to issue a proof of correction certificate to vehicle owners upon completion of emission related recall work. Please fill out one of the campaign completion forms for each owner that has this campaign performed. Instruct owners to keep the certificate unless they are requested to mail it to the DMV. A sample of the form is shown.
NOTE : An initial supply of these certificates is included with this bulletin. Additional quantities are available from Dyment (1-800-247-5321) under part number CAEMRC98 at no charge.