Hello Chad,
It sounds like you have the early Becker Stereo Radio. The early stereo units had separate amp box with jacks for the two stereo speakers. Later Becker Stereos did not require a separate amp box when electronic pars became smaller. To fit this radio in the smaller 230SL oppening a front radio mount and faceplate assembly are required as used on the original TR versions.
This can be done easily and becker will provide the correct parts including the early knobs to match your car if needed.
I have done some posts with pictures, you may want to "search" them up. I am pressed for time at the moment and will try to help again later.
Becker of NA has published a nice Becker Vintage Radio Pamphlet which we distributed at the Tech Session, I have some left if you would like one. They show the different vintage knobs faceplates etc. which they still supply for our old Becker Radios. Ed Ebel at Becker is very helpful also. Always tell him your one of our group. He has been very supportive.
Yes Becker Stereo Radios were offered in the latest W113 cars. I have the Becker layout showing the dual speakers, and location if needed.
Back to you ASAP,
Joe Alexander
Blacklick, Ohio