will the 03 and up stock stereos plug up to 02 and lower trucks? any modifications needed? how about heater and AC controls?

the radios out of 2002 trail blazers are the same as 03 and up trucks, and the harness is diff. as for the ac control i dunno, and i dunno if you can mod the 02 and 03 radios

The plugs on the 99-02 trucks and the 03-05 trucks are not the same at all.

You might could look at both harnesses, and see if the wire colors are the same and if they are, splice in the new harness at the same time.