The problem is the difference in intended use for the audio output points.
On the SP3, 26/27/28xx 'Garmin Motorcycle Cable' and the Quest 'Garmin Motorcycle Cable', the 2.5mm voice audio output connection is mono and is designed to connect to a speaker output only. Connecting this output directly to an off-board external amplifier is not supported, will cause grounding issues with the 1800 and can damage the audio output circuitry of the GPS (which is a non-warrantied repair).
On the units that are capable of outputting stereo music (2730, 2820, 376, 378, 478 and the Zumo) they provide a 3.5mm stereo output jack. On the Zumo, this is the only physical audio connection point - the other units still also provide the mono output capability, but it's disabled when the unit detects a cable plugged into the stereo output jack.
Bottom line - connecting the 2.5mm mono output jack to the bikes 3.5mm stereo input plug was never a design intent of the Garmin system, nor was it the intended use of the AUX input to the bikes audio system.