The new ECM did the trick. The swap took 5 minutes. Once I plugged the new one in, I turned on the ac and both fans fired right up. I drove it uphill for a few minutes and they turned on low, then high, as they should.
In case somebody else ever has this issue, this was my process:
0. CEL checked, p1520 - Coolant Fan Relay 1 High Input.. Upon further inspection the drivers side (main) radiator fan wasn't working with the ac or high engine temps. Great.
1. Swapped the problematic relay with a known working one. The problematic relay fired up the headlights as they should, and the working relay didn't get the fan to work.This ruled out a faulty relay
2. Jumped the relay connections to rule out a broken fan. The way you do this for the 98 Forester is you locate the fuse box in the engine bay. Read the label and find the faulty relay, in this case it's in the AC cluster, labeled "main fan". The relay has 4 connections, in sort of a T shape.
If you're wondering the 4 connections for this relay are as follows, from the bottom of the "T" (numbering is from the wiring diagram I used):
25) Line directly to the fan
26) 12v fused from battery
27) Signal from ECM
28) Line to AC fuse in the car.
In this case, you jump 25 to 26 giving the fan power from the battery. In my case, the fan fired up, so that ruled out a broken fan, and also confirmed the wiring from the relay to the fan is fine and to the battery is fine.
3. Check the continuity between the ECM and the relay. (The ECM is located under the pssenger carpet u see a metal kick plate. 3 bolts and a nut takes off the kick plate to access it.) I plugged a 3ft length of wire to the 27 lead and used my volt meter to check the red and light-blue wire on the ECM (third from the left on the top row of the ECM harness (which is plugged in in an upside down position on the ECM.) The volt meter hummed confirming the wiring between the relay and the ECM is fine.
4. (Credit to cardoc on subaruoutback.org for this info) Back-probe the ECM to check for proper voltage. Use a paper clip and carefully stick it into where the red and light blue wire comes out of the ECM harness.
When the radiator fans are triggered by the ECM because of high temps, the voltage from the ECM should change between 0-10v. If there's a change, ECM is fine. If not, further testing required.
Turn ignition to off and remove BOTH fan relays (sub fan and main fan), turn the key back to on and retest. If you get a reading more than 10v, the ECM is fried.
If not, test the resistance on the problematic fan relay itself, less than 1ohm, replace the relay and the ECM is likely fried. If more than 1ohm, test the other fan relay in the same way and replace if needed.
If you pass every test, replace the ECM and see if that fixes things.
I personally got a high voltage with the relay out (hence the p1520 high input code) so I replaced the ECM and voila!
Wiring diagram (labeled page 60, actually page 49 of the PDF shows the fan wiring):
Testing procedures (cardoc's post on page 2, right at the top):