1999 chevy s10 not starting

sounds like bad battery, also check for parasitic drains on electrical system. Batteries usually start going bad in summer or warm months and when it is colder there is a larger draw on battery current.

test and check battery--battery terminals especially positive --starter

Starter Relay
Center rear of engine compartment

you're correct the 98's come with an EGR valve and the 99's don't.

I think you have a bad connection or bad starter relay

think your starter motor is no good or the battery is flat ,try charging the battery for a couple of days as well

You Probably have a Bad starter motor, or Starter solenoid, Take the starter, and get it checked, most autopart dealers will test it for free.

Yhea they can be a tight fit, really tight if its 4x4!
Your local parts store should be able to test the starter while it still on the truck. You can hook up the vat machine and run a starter drain test! Make sure the cables are tight and clean before you go too far. A jump start being needed usally means that the battery or the cables are bad.
Make sure they test the battery when they test the starter!

heres a quick test you can do start the car and with it running take off the positive battery cable. let me know the results.

Dan , check the wiring harness where it droops down on the pass side near the radiator. the wiring always gets corroded here and all kinds of things can happen. open up the harness and check the wiring. repair the obvious wires and then tug the rest of them. also check the fusebox in the engine compartment for corrosion and poor connections to the relays. there are two grounds on the front of the engineand most importantly one major ground on the lowest tarnsmission bell housing bolt/stud
near the starter. all of these were common with these vans.
also chech the fusable links at the starter-baisically pull on any suspicious looking wires you see might be corroded or have a "BUMP" in them wich is usally a ball of corrosion.
good luck, wish i was there to help