We run into the situation when GROM users connect GROM to the back of the stereo, but forget to disconnect the external CD changer. Note that in order for GROM to work in the CDC (CD Changer mode) external CD changer must be disconnected. The external CD changer can be located in the trunk (such as in BMW), under glove box, arm rest area, or even at the dashboard above or below the stereo.
Make sure to disconnect any external CD changer if present.
NOTE that internal (build-in) CD changer will remain working.
All above applies to the GROM kits that use CD changer emulation (CDC). The kits that use SAT (XM) emulation such as for Nissan/Infiniti with SAT/CAT button do not need to have external CD changer disconnected.