Bought an 89 firebird formula as a daily. It’s a 305 tbi with auto trans. Engine was locked up but I had another 305 tbi laying around from my 88 TA swap. Swapped it in. Engines in but have no power to any dash/headlights/parking lights. No fuel pump cycle when the key is turned on and no SES light either. The car does crank. The fp/ecm 20Afuse was blown when I got the car. Put a new fuse in it and it is currently holding. I’ve replaced 2 of 3 fusible links at the starter (replacing the last in the am) and 1 of 2 at the distribution block by the battery/radiator (it was shot) will replace the next in the am.
The odd thing is the turn signals work and the factory radio gets power though it doesn’t do anything (just the time) No power window/lock/mirrors. Grounded on the head and both grounds on the rear on the heads are in place. All fuses at the block appear good. Fresh-ish battery bought last summer out of my 4th gen being used.
Looking for ideas on what to check next following the next couple fusible links being replaced. Would a bad ECM cause all of the power to go out in the interior?

 

The fusible links are what powers different parts of the car and fuse box. Sounds like the 1 you needed to replace is the one you haven't yet. You can do a simple continuity/wiggle test with a test lightto make sure they are good and still work

 

Your ECM controls the engine's associated bits, and the lockup function of your torque converter.
It has absolutely nothing to do with interior stuff, your headlights/taillights, power windows/mirrors/locks, and any of your other non functional electrical stuff.