I dont have a box under my seat and i have keyless entry option. I bought the truck used and it didnt come with remote but started doing some research and if you follow this code and your locks respond- you have the option:

** from another site, these are the programming instructions

The vehicle should be in the following conditions
1. The key is not inserted in the ignition key cylinder.
2. Drivers door is open (other doors are closed)
3. Drivers door is unlocked
Insert the key into the ignition key cylinder, then pull it out

Within 40 secs.
Press the door lock control switch (for manual operation) 5 times to LOCK/UNLOCK at approx. 1sec. Interval
1. Lock then unlock
2. Lock - unlock
3. Lock - unlock
4. Lock - unlock
5. Lock - unlock

Close the drivers door, then open it. Again press the door lock control switch (for manual operation) 5 times to LOCK/UNLOCK at approx. 1 sec. Interval
1. Lock - unlock
2. Lock - unlock
3. Lock - unlock
4. Lock - unlock
5.lock- unlock

Insert the key into the ignition key cylinder.
Turn the ignition switch from on to lock at approx. 1 sec. interval 1 to 5 times to select the mode.

Add Mode
ON - LOCK change occurs 1 times

Confirmation mode
ON - LOCK change occurs 3 times

Rewrite mode
ON - LOCK change occurs 2 times

Prohibition mode
ON - LOCK change occurs 5 times

Pull out the key plate from the ignition key cylinder
When add mode or rewrite mode is selected.
Body ECU automatically performs the LOCK-UNLOCK operation once or twice at 1 sec. interval to inform the operator that either the add mode or rewrite mode has been selected

LOCK-UNLOCK occurs once indicates that add mode has been selected
LOCK-UNLOCK occurs twice indicates that rewrite mode has been selected

Press the lock and unlock switches on the transmitter