i think to try to answer one of your questions:

if u had all your speakers hooked up to a 4 channel amp in the trunk and u sold it. and now u want to use your HU's amp, then yes u will have to run wires from those speakers to your HU.

also from looking at the picture i can only identify the blue as the remote lead wire. red is either ignition or constant 12 volts. usually ignition. and the yellow is usually constant 12.

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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