I see you found the Curt Powered Tail Light Converter # C59236 which does require you to splice into the tail light wiring on your 2019 Mercedes-Benx GLC300.
If you wanted a harness which does not require splicing that would be the ZCI Circuit Protected Vehicle Wiring Harness # 119250KIT. It has the sensors which detect power going through the wiring rather than pulling power from the wiring. view full answer...