My dad bought this truck and loves it other than that there are no park lights. The brake lights and turn signals work but he has not been able to find the reason for the park lights.

I just need a push in the right direction. What things should we look for? I've posted before without any answers. I know that I don't have much to go on here but just any indication of what I need to start with to find this problem would be great.

Do the parking lights come on with the headlights on?

No. They don't come on at all. This includes the lights on top of the cab.

I'd suspect the light switch but check this manual, I have downloaded it and it's good.

http://pdftown.com/Dodge-Ram-2001-Service-Manual.html

I'm downloading it as I type this. I'm sure that this will help.

Replaced the light switch. No change. Further suggestions?

Do the instrument panel lights come on?
Checked fuses?
Are the cab lights stock?

The answer to all of these questions is yes.

Did you check the fuse panel under the hood? There's a little 15 amp fuse in a row of little fuses. There's a diagram under the top of that panel.