Not sure on the '98 but I just put an alarm in my '02 and hooked up the door trigger wire to the yellow dome light wire at the ctm. It's a positive trigger, meaning that there is no voltage when the dome light is on, and 12v+ when the dome light shuts off.
My alarm has a 60 second delay option for arming and the dome light shuts off before then, so it works perfectly.

 

Here's an option I did under my garage and should work here. I went to harbor freight and got on of those driveway signal alarms. I'd put it under the dash. Paint it black or something. When some one enters your truck the remote on the other end will chime and you'll know there in your truck. Just a thought. Think mine was less than 20.00.