Have a buddy hold the tire rack in place while you mark the holes. Its easy but mounting to the body is very weak. Make sure its always secure before you drive. Mine came loose with a 35" spare tire and caved in the side of the truck.
I then made a tube frame mount that welded to the bumper. No more problems.
I don't know about the 90's butsome of the 80's came with stiffening hardware welded on the inside of the right rear skin. If there was no spare tire the bolt holes were filled with bondo and painted. Take off yout right tail light and see if you can find a brackets in the approptiate places with what looks like glop hanging out. If so, all you have to do is clean out the bondo and run a tap thru to clean the threads. You definately need the reinforcement hardware.
A body shop told me that if it didn't come with an outside spare mount that it didn't have the stiffener inside the body. I have also seen new rear quarter panels for sale that have a different part number for a right rear for use with a spare mount.
if it didn't come from with one from the factory don't add it! the support is not there, have one made that mounts to the bumper. you need the support behind the fender or it will come off.
Well some of them did in fact come with it just as I said . I have just such a truck. However, jfrbronco is right, if you looked behind the tail light and there is nothing there but smooth skin, you need it and [-X don't try to bolt to the skin. It will rip thru. There is also a stiffiner that goes inside the tail gate.
I don't happen to like the big ugly thing hanging on the back of my truck. I have the internal tire rack.