You need this or something like it in order to use the jack in the back for mp3 / ipod / aux input:
http://www.ebay.com/itm/151345044531Note that the plastic cap has two metal pins in it. Those pins are completing the radio out to amp in connection. If you remove the cap you won't get any radio sound. When you plug in the Becker Cassette deck it completes the radio circuit if you don't have a cassette in the deck, or connects the tape out to amp in if you DO have a cassette in the deck.
Similarly, you need a switch box on the other end of the cable if you want to run a 1/8" audio in to the radio. This is because the switch box needs to have the smarts to connect the radio out to the amplifier in when you DON'T have a aux in device plugged into the switch box.
You will find cheaper versions that don't have the switch box, just a din plug and wire and an 1/8 audio on the end. Since access to the back of the radio isn't easy, that means you are giving up on the radio capability entirely if you go that route.
As other poster said, the recessed spade connectors are switched power for a power antenna.