In my car the cracking followed the revs and was traced to the alternator. It turned out that most of the grounds on the engine and Alt were bad. I replaced all ground straps and hope the cracking will be gone.
Furthermore the best thing you can do is pull power and ground directly from the battery, with a sufficiently fused line. If you then Isolate the radio from the car body, you will not get any interference from the car as it is completely separated from oneanother.
When I reinstall all my stuf in the car, I will take this approach. One ground to the back one positive to the back, gold plated splitter blocks in the line in the front and rear to connect it all to and insulation tape around the head unit so it doesn't contact the car body.
It's a bit more work, but should be worth it in quality terms. It also mkes trouble shooting the rest of the cars electricals easier, as you can delete all audio stuff by simply pulling the main fuse.
Until now, my radio was wired to the original wiring in the car by use of scotchlocks and stuff. This lead to car troubles, that eventually were traced to the radio setup. In the future this will be a one second check instead of hours.....
Just my view...
First time rolling restoration, 66 289 nearing completion.