Refer to chapter 8, page 25 of the German operating instructions, available as a download for your "PIONEER X-CM56D". There you will see that there is a 3.5 mm mini jack "AUDIO IN" on the back of your unit. Connect this jack to the "AUX OUT" jack of your "YAMAHA WXC-50". To do this, you will need a suitable audio cable, stereo jack 3.5 mm to 2xChinch-RCA, available at any Media Markt, Interdiscount or similar. etc.
Have fun!