Looks like you don't need an impedance splitter, just a signal splitter like this:Can you link to an example of an appropriate splitter? car audio is totally new to me. Hell, I don't even know how to spec an amp.
https://www.amazon.com/Female-Y-Cab...no+to+stereo+splitter+rca,aps,202&sr=8-2&th=1
You could wire a male RCA plug to your existing mono out wire (use red for + and brown for - ) then plug it into the splitter, then the splitter goes into the speaker level amp inputs (make sure your amp has line level and speaker level inputs). Some amps don't use RCA for speaker level inputs, in this case you could just make your own splitter cable by taking regular speaker wire and cut 2 pieces, one each for the amp input, then wire the other ends to a single wire that goes to the Blaupunkt (3 positives and 3 negatives connected together).