So what you want to do is replace the 'el cheapo' in the 82 300SD with the working 6000EL that you have? And the plug on the 6000EL antenna doesn't match the one on the car wiring harness? If this is all correct, what are you running the antenna from, an original Becker radio/cassette or aftermarket? And does your car have a dash mounted 'antenna switch', designed to bump it up/down in increments?
Sorry for all the Q's, but we can probably help better/easier with the complete picture.
Meantime, here's a great shop that might be helpful. If you look under make/year, they sell several adapters that look like what you want.
http://www.antennaworld.com/2004/departments/amfm/techinfo/mercedes/mercedes_auta6000el.shtml
However, if it were me, I'd rather save the money and hassle and just wire your working 6000EL into the harness to go up and down with the radio on/off and skip the complexities of the dash antenna switch (if you have one, that is).