i'm guessing since the aftermarket deck doesn't have a wire for a dimmer, there should be a setting on the deck for that purpose, just don't even connect the dimmer wire, just connect the illumination wire to the illum wire

And, you could prolly put in fifty more decks, and still have the exact same problem, damage has indeed been done.

Do the rest of the lights dim, a/c lights, all those heater selector lights, etc?? If those lights dim, then your dimmer circuit should still be intact,

Also, have you pulled your instrument cluster and checked to see if the bulbs are burnt out?