stilako

Generally good advice. I ran onto something else a couple of years ago, with something in the dash. Can't remember if it was speedometer circuit or cruise control. The problem turned out to be a cracked joint in solder on a circuit board. Very hard to find, and was eventually spotted with a strong magnifying hand-lens. The solder used on the old bikes (and may still be used) was a very hard mix. Resoldering with rosin-cored solder (don't use acid-core) fixed it. I use a butane-powered pencil solder heat tool, which also keeps the possible static of electrical guns and pencils away from such circuits.

You have three pieces of equipment involved: GPS, Adapter, Cassette player. If you can, try eliminating one at a time by using other equipment. E.g., if you can put the adapter in a household cassette player, and play the GPS through it, get stereo, it's your cassette player. If you still get mono, the problem isn't with the cassette player, but with either the adapter or the GPS. Isolate the unit causing the fault, and go from there.

This is much later, but I've been looking for a noise answer. If your volume control must be very high to hear on the intercom, check the MUTE button. On mine, it's mounted above the left hand-grip. If that is ON, you will be able to hear on intercom, but the volume will be low. If it was on, and you push it, it will go off, and you may find your ears getting blasted. I have a Sirius sat radio on my '87 VR. I bought their "FM Direct Adapter". It works very well. Originally I purchased one for my truck, because when in Vancouver, I couldn't find a portion of the FM radio dial that wasn't being used by an FM station. The Direct Adapter goes in the line between the external antenna (on the back of the VR) and the radio tuner. When the Sirius receiver is turned on, the Direct Adapter cuts the circuit between the external antenna, and sends the signal from the Sirius directly into the VR's FM receiver. You still have to set the Sirius output and the FM receiver to the same FM frequency, but there is no interference from commercial FM stations. Or AM for that matter.