Hi - i know the hum you are battling very well. It will come and go depending on what components you add to the system, what components you subtract, and how you handle the grounding and powering of the speakers. I hate to say it but in many cases the crutch field adapter may not get rid of the hum.
To try to give you a little bit of exp from my turns - ill give you a little background, on how i handed my 95.
Truck came to me with pop-out clarion unit installed, looked like a best buy install. Speakers were pretty much shot, original amp still in place, and factory subwoofer was installed. Whoever did the install cut into the factory wiring and wired in RCAs into the factory wiring to plug into the clarion.
Iteration 1 - i changed out the speakers... had a dramatic improvement in sound, but now subwoofer was really shot.
Iteration 2 - I added in a self contained amplified sub off the clarion and removed factory sub, kept factory amp. Heres where the Hum came back and did not leave, even when disconnecting the new sub, hum still there. The ONLY way to stop it was to add a 4 channel isolator... i got it off a company called monoprice. Worked like a charm!
Iteration 3 - I hated the clarion. decided to remove it clean up the rats nest of wiring, and i wanted apple car play so I added in a pioneer flip out. Now even the isolator did not stop the hum, i ran all new power and ground wires directly to the HU and still no luck. I finally broke down and said F it, i eliminated the factory 20+ year old harmon kardon amp and ran direct speaker wire to the doors and rear. Problem solved, hum gone.
I really like the clear sound of new updated components. Unfortunately my center dash did not survive, had to make one work from a D2 and a dremmel. Its fine... good enough for me. Be careful with yours.