I took my '87 300TDT to a reputable radio install shop to see about replacing my "working" Becker Radio. After an hour of checking out the car, they informed me that they would not try to install anything!
After picking up car, I noticed that the power wire to the radio is only hot when front doors are open. WOW - Did they hook something back up wrong.
Please help me - I look funny driving down the road with the door open.

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1987 300TDT 165K MILES

Almost sounds like someone spliced into the dome light circuit for the positive battery connection. Maybe they played with the wiring harness for awhile but were not having any luck figuring it out - so they quit.

I don't know why they would do that to your wiring harness - but I can see how someone (not too bright) randomly searching for a positive battery connection might pick the dome light circuit - while they had the door open working on the car.

Good Luck - I recently had a very good "do it yourself" experience installing a Kenwood KRC-309S cassette deck in an '82 300D - it requiring pulling out the previous owner's DIY harness confusion and a complete harness rewiring using the Kenwood-supplied items.

I had the same problem with my 420SEL. Only, the mechanic fixing my A/C was the one that wired my new Kenwood deck wrong. I took it back to where I had it installed, and they fixed it for free, even though it wasn't their fault.

The constant power was apparently wired wrong, along with the main power. Bother were wired into the door switch somehow. When they fixed it they wired the constant power into one of the horn switches i installed under the dash that's hard-wired to the battery. I found that slightly funny, but it works.