Does anything flash when you lock/unlock?
Assuming Isuzu wired the keyless like everything else in the car, the lights would be ground switched, so there should be an illumination wire you're looking for in the wiring harness. To narrow it down, I noticed the interior lights that flash (all the dash lights except the radio, which is wired a little different-not important here) will flash according to the setting you have on the dimmer switch. I just went out and tested this theory on mine, which showed this is true. The reason I did this was to determine which wires you'd be looking for. The wires connected to the dimmer are red with a green stripe. Now my thinking is the red with green wire should run from the dimmer to the keyless module and the light switch on the turn signal stock. Therefore, the lights light up at the intensity set by the dimmer when either you turn on the parking lights/headlights or unlock/lock the doors.
The module I briefly mentioned is a black (in my car it is-some other have mentioned it being different colors) box located inside the dashboard to the right of the glovebox. Simply remove the two screw on the glove compartment hinges at the bottom and then open and pull out the glovebox. When you look to the right, you see a box mounted parallel to the ground and one perpendicular to the ground. The parallel one is the keyless module; the other is the wiper relays (it's marked alarm on the outside, but really it's only the chime when you leave the keys in the ignition or you leave the lights on). There are two screws on the bracket, but from my experience someone smart at the factory wrenched these screws in. I took the easier route and popped the tabs on the mount to release the box from the bracket.
From here I'm not fully sure what the correct course of action is, but this is my thoughts. You should look at the wiring plug/harness/whatever you call it coming into the module (disconnect the module first of course). Try to determine if there is a red w/green stripe wire coming into the harness. If so, I'd use a restrictive ground-such as a test light-to ground out that wire. If I'm correct, this should turn on your interior lights. This would at least eliminate the possibility that your wiring is bad somewhere, but would give rise to the possibility that something's wrong with the module. Likewise there should be another for the dome/courtesy lights. These should be on another wire simply because they don't have the same action as the dash/parking lights-they come on but don't flash when unlocked and don't turn on when the car's locked.
Unfortunately I don't have the factory service manual (don't use Ebay they ripped me off) so I don't actually have the wiring diagrams in front of me, otherwise I would have a lot better idea of what to look for in this case. I know for a fact that the lights that are illumination (gauges, switches, radio, a/c controls) are grounded through the dimmer, but I'm not positive where the turn stock switch for the lights falls in the circuit.
I hope this helps, but I do realize it's sort of useless without the proper wiring information.