Are there any modifications that have to be made to fit an aftermarket stereo in a 98 Blazer?
Do I have to unbolt the factory radio support bracket?

Otherwise I'm going to get a dash kit and vehicle harness and probably put one in.

Are they all the same harnesses or are there different ones?
And is there any inline amps or is everything powered from the stock unit?

 

you will also need an antenna adaptor. you will have to break a small part of the dash out for aftermaret to fit nicely.
Clicky

For actual directions for removal and installation of an aftermarket radio, you can download the pdf (adobe) file from http://www.installdr.com/InstallDocs/GM/PDF/460001.pdf

The parts that you will need are as follows, all metra part numbers, 993300, (dash), 70-1858 (wiring) and 40GM-10 (antenna).

There should not be an amp in the truck.