the 6.5" will not fit in the rear deck, it uses 5.25" round (have em in my 83 300d-t)

also the 4" on the drivers side wont fit with the cover down, i can barely get the cover down on my drivers side front deck and i have Blaupunkt 3.5"s lol

to answer some of your questions the original lines do handle the sound but not as good as good as it could be, if you have the time def rewire it all.

best path for RCA is down the side of the car under all your carpet. As for the powerwire run it off your battery and through your firewall (one of those rubber plugs will pop off. Also remember to run your power wire and RCA down oppisite side otherwise you'll get nasty static.

any more questions just hit me up.

My car 83' 300d turbo front deck: 3.5" Blaupunkt rear deck: 5.25" Infinity trunk: 2x 12" Xplode and 1000w amp

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83' 300D Turbo ~ 168k (process of getting a new engine)
90' 300TE ~ (SOLD)
81' 240D (SOLD)
84' BMW 318i (SOLD)
91' BMW 318i ~ 120k
88' BMW 325i (SOLD)