For those having trouble eliminating alternator (or gasser ignition) noise, here's a suggestion. I did this about 5 years ago on the Blazer & it worked great.
Run a piece of coax from the battery to the radio. Look for one that has a decent size center conductor, preferably braided, I think I used RG8-M. Connect the center conductor to the positive battery terminal and to the positive lead on the radio, as close to the radio as possible. Now, connect the outer shielding of the coax to the negative battery terminal, and leave the other end open. Finally, connect the radio's ground wire to a good ground, and don't forget to install an inline fuse.
Hope this helps someone.
Pete
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'97 2500 CC 4X4 5spd Sport
'83 K5 Blazer 350 TBI (ex 6. 2), 39. 5 TSLs, 3" lift, Dana 60/GM 14 blt, Detroits