Be very cautious about how you connect to the amplifier and what you connect to it. No provision is allowed for a 3 speaker set up. Only 2 or 4 speakers can be used. Unless you have one speaker in the front and one in the rear You'll have to forego the use of your dash speaker. Please refer to the picture below for reference. (these are factory drawings and literature).
Mercedes never intended for the Pagoda to have more than the one dash speaker. However, Becker did offer a stereo set up for them for subsequent installation (though, not approved by Mercedes). In order to correctly use this offering one of the four connectivity diagrams below must be used. If the connections aren't adhered to you'll have an impedance mismatch. In a typical Becker 4 speaker set up each speaker is a 4Ω speaker. The 4 inch speakers are rated at 20 watts each. Essentially, if the total impedance in a series circuit with both inductive and capacitive elements is less than the impedance of either element separately, then the total current in that circuit will be greater than what it would be with only the inductive or only the capacitive elements there. With this abnormally high current through each of the components, voltages greater than the source voltage may be obtained across some of the individual components....in particular the germanium transistors in your radio that are nearly impossible to find now. In these particular radios leaving out a component is futile to the radios well being long term. And as far as connecting two channels into one speaker.....don't do it for the aforementioned reasons. One or two speakers for left and right channel only. And if using only two speakers for left and right no fader potentiometer is to be used.
Regarding originality, the originals didn't have screw mounting holes in the front of them. (see picture below) The caption underneath the picture translates to "Stereo speakers in the front, from mid-1971 onward"The actual speaker would screw to the car, and then the cover would snap on over the speaker. What you've found is what was offered on later cars, but should work just fine. As far as where to mount them, there is literature from Becker that gives measurements on where to mount them and to drill the screw holes and where to cut out the large hole for the permanent magnet.
