Sounds like you are going from an internal amp head unit to an external amp head unit. The plugs on the amp itself will match the radio from your '98. The harness between the amp and radio won't match anything else.
You will need the amp from the '00 for it to work in your truck.
-Charlie
I have the amp in my 98, sitting in my lap. The harness i clipped was also on a amp.
I