I've been having issues with my '90 YJ. It had intermittent starting problems after I replaced the intermediate steering column.
Sometimes it would start just fine and sometimes no starter but the dash lights & heater worked (I could jump across the starter with ignition key on and it started fine).
Now, no dash lights, no heater, no ignition... no nothing (starter turns over when jumped but won't start). I'm thinking ignition switch. Does that make sense? I've read a few write ups on replacing the switch so I'll probably do that as a first step.
First, get a helper and a volt meter. Check the fusible links to make sure you're getting power to the fuse block; sometimes those burn partially. Also see if the power from the switch to the starter relay is working. Green wire, as I recall.
take the ignition switch off the column, manually work the switch with a small screwdriver. the spring loaded position is the start position. if that works then it is a simple matter of adjusting the switch
May be a long shot but easy to check... I would look at the ground wire off the battery to the block. I had the same symptoms a while back and found that it was exactly that. Got new battery wires and terminals and installed and cleaned the area it attaches to the block for clean solid contact and voila.
Put the column (w/ new switch) back together and looked at the starter relay (bolted to fire wall).
One wire (off the main pwr post) was smoked... looked like an old problem but jumpered across it anyway and got dash lights (also got pwr to starter relay).
That seemed encouraging so I shorted across starter... she started and ran perfectly.
Ordered a new starter relay (everything up to it else seemed fine) since that is the likely suspect.
No disrespect but it's often less time consuming (and cheaper) to toss parts at a problem than spend days tracing/diagnosing an issue... especially if the parts are cheap and time is limited (or you're working in the driveway and it's below freezing).
I believe I've found the problem...
After I repaired the smoked wire (fused it w/ small fuse - 10A), I had instrument lights and, therefore, pwr to ignition coil.
It started fine and ran great (as always) but when I put the heater on full, the fuse pops (much better than burning more wire) leading me to suspect that everything (dash-wise) is on the same circuit.
Well, last summer I did the blower motor upgrade (older blazer motor fits great and delivers a hurricane of wind). I'm guessing the new motor pulls lots more current than the OEM and overloaded the circuit (don't know why junction box fuse didn't blow - maybe too big a fuse), smoking the wire. So, I ran a separate circuit (with relay & fuse) for the blower motor and haven't had any problems since.