I have been working on the research for tach install and have found many for the older trucks with success and slime for the new. Somewhere I saw info and then could not find it again, but this is what I did and it worked.

2000 4.2 with no factory tach
Used Autometer 8,000 rpm w/shift light

took off battery lead
unbolted main wiring harness from computer
unclipped black wire shroud
take apart harness connector
-three piece unit: plastic cap, rubber insulator seal, plug in wire holder
-unclip cap and pull back and get slack about 1/2 inch between cap and plug in wire holder, will then see the blue rubber seal
-find #48 spot with no wire comming out of cap between #47br/pk and #49lbl/b and drill with 3/32 bit and remove the little stub 1/8 " long on back side of cap
-buy green 18 gage wire and strip back 3/4 inch of insulation and push through cap and blue seal and into (plug in wire holder) until wire just reaches the end and push cap on carefully while keeping wire depth correct
-zip tie green wire to near by wires to secure from pullout, use several wires and zip ties
-replace black shroud and push plug into computer and tighten bolt

Green wire is now the tach signal wire for 6 cylinders
I cut brown wire on the autometer tach to set it for 6 cyl. setup
I installed fuses on green wire 12" after plug with 30amp
-red tach power 10 amp
-white illumination wire uses factory fuse
-black ground to metal body or frame

Tach works with no problems
I replaced the too-bright shift light bulb with two LED's that are green, I will post pictures soon

 

damn i guess thats why i could never get mine to work i never would of thought that there would be something blocking the number 48 pin in the computer. Everyone just said to stick a wire in there.