i believe these cars speakers are wired to the fader in the middle. i would advise u to run new wires to each speakers. so if that is done with u only need to find constant 12v ignition and ground. and remember black doesn't always mean ground on these cars. sometimes brown is ground.

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198? 300D ???K - Future DD
1989 300TE 285K - traded in '12
1984 300SD 186K - Sold in 08
1978 300SD 330K - Died in 05