Typically there will be one deep, wide groove across the outer diameter of the harmonic balancer and that's the timing mark.
One way to figure out what's what is to pull all the spark plugs except #1 (pssgr's side, front plug). Then, rotate until you feel compression. As you get to TDC, compression will peak then slack off, at that point the timing mark will be pointing to Zero on the timing scale. I marked mine with white paint to help it stand out.
From there, position the distributor such that the rotor is pointing to around the 7-o'clock position as you're facing to the rear of the vehicle. Whatever contact in the cap the rotor's pointing to at this point becomes #1 cylinder.
If the rotor is not in the 7-o'clock position, either remove the distributor and reorient the rotor, or leave as is and wherever the rotor points becomes #1 and you follow around the cap with the other plug wires.
Firing order is 1-2-3-4-5-6, clockwise, around the distributor.
1-3-5 on the pssgr side, 2-4-6 on the driver's side.
Don't forget to unplug the inline connector on the ESC (Electronic Spark Control) wire when you adjust timing, not possible to accurately set the advance without unplugging this wire. This wire is the only one in the center console (check under the front tray of the console first) with an inline connector and is usually white with black stripe.
You'll find if you set advance at around 13 deg BTDC she'll run a bit peppier.
HTH & G'luck.........ed
'90 3.4 Troop LS;
'89 Troop RS (Exhaust Valve Challenged), now gone to a Good Home!
Yes, I am a Trooper-Holic!!!
Keep On Troopin' !