Just some thoughts. The average MP3 player output is roughly 60mW. I would assume that the CD players those cassette adapters were originally designed for would likely be roughly the same. I'm not familiar with that particular radio's specs, but most of 2m & 2m/70cm mobiles output a couple watts. I'm not so sure that cassette adapter could handle the additional power, especially one with considerable age/use. I would also assume that driving a couple watts of power into the stereo's pre-amp isn't really all too helpful either. I would think you'd have to drop the Icom's output wattage if you were going to use the stereo no matter which way you connect it. I'm about to install a new rig in my Subaru & the extra $40 for the external speaker seems to me to be a better deal than the headaches of a work around that may or may not end up in creating new problems in other areas.
The whine does sound like an ignition/alternator problem, but you also noted that you stopped using the adaptor & are back using the Icom's internal speaker. That makes me believe that there's another problem somewhere after the radio i.e. the adapter and/or the stereo.
Just my $0.02.
Last edited by AK7OZ; 05-30-2012 at 07:52 PM.