I have a '83 SD that had a Kenwood cassette deck already installed when i dought her. Thankfully whoever installed it didn't hack the harness. I replaced the Kenwood with a JVC cd player. It was just a mater of replacing wire for wire. The fader was left in the loop and it still works. I still need to redo the speaker wiring as it has an issue now. I don't remember the exact wire color but your '82 SD should already have a constant 12vdc for memory and for your radio clock. This should help out
STAR TekInfo 12vdc in the harness.